Magicento

Using PHPStorm 6+ (*) you will get code completion for factories automatically (just use Ctrl+Space like always).
This is useful not only for code completion but also for "go to declaration" feature (just use Ctrl+B / Ctrl+Click / middle click (windows)).

It works for $this->helper() inside templates and $this->_factory->getModel|getHelper|getSingleton|getResourceModel too

* If you are using an old PHPStorm version, you can have code completion using "add @var" action:

with the cursor over your $variable use Alt+M (Option+M)

PHPStorm 6+ required (with older versions you can use Alt+M > add @var $this on any .phtml file).
Make sure you have enabled this option inside Magicento settings.

Note: sometimes is not possible to find the correct Block for the current template (for example when the template is not assigned with the layout.xml but is hardcoded inside the Block class, or when you have wrong package/theme in Magicento Settings)

Just use Ctrl+Space after the "class " keyword

It works for controllers too

Note: this could be a big time saver because your class names won't have typo errors anymore, also is very useful when you need to refactor some module name.

Use Ctrl+Space after the "extends " keyword.

Use Alt+M when the edit cursor is inside the class you want to rewrite and select Rewrite Class or Rewrite Controller.

Then select the target module where the new class will be created.
And lastly you can change the default class name (proposed by Magicento) for the new class.

When you are rewritting a controller the dialog is a bit different:

The prior dialog will generate a class and xml like this:

Note: the only requirement for these actions is to have a valid target module already created, with a config.xml file (config.xml can be empty, Magicento will create the correct nodes)

Use Alt+M anywhere inside any opened file and select Create Module

The options are self explanatory
The "Group" field can be empty and it will use the default one (NAMESPACE_MODULENAME), in the image above the default group is "magicento_test" but it was changed to "mtest"
You can choose if you want to generate the nodes and folders for the blocks, models, helpers (in this case it will create a Data.php file too) and installers.

These are the files generated by the prior dialog:

Use your "Go to" shortcut (by default Ctrl+B / Ctrl+Click) on any string representing a factory to jump to the class for that factory.
This works everywhere:
Mage::getModel('[CTRL+B HERE]')
<block type="[CTRL+B HERE]" ... >
$this->_init('[CTRL+B HERE]')
etc

Use your "Go to" shortcut (by default Ctrl+B / Ctrl+Click) on any relative path to some file.
For example
<block ... template="[CTRL+B HERE]" >
<file>some_layout_file.xml</file>

Use your "Go to" shortcut (by default Ctrl+B / Ctrl+Click) on any string representing a block name/alias or handle name.
For example
<reference name="[CTRL+B HERE]">
<update handle="[CTRL+B HERE]">
$this->getChildHtml('[CTRL+B HERE]')
$this->getBlock('[CTRL+B HERE]')

Use Alt+M when the edit cursor is inside a Block class and select the action Goto block definition in layout

Note: sometimes Magicento can't find the <block> nodes where the Block class is used. This happens for example when the block is created from PHP instead of using the layout (adminhtml grids for example).

Use Alt+M when the edit cursor is inside a Block class and select the action Go to template from block

Note: sometimes Magicento can't find the .phtml file/s where the Block class is used. This happens for example when the template is assigned with PHP instead of using the layout.

Use Alt+M when editing some template (.phtml) file and select the action Go to Block

Note: sometimes Magicento can't find the Block class reference by $this in the .phtml file. This happens for example when the template is assigned with PHP instead of using the layout.

Use your "Go to" shortcut (by default Ctrl+B / Ctrl+Click) on the event name inside any Mage::dispatchEvent call.
Mage::dispatchEvent('[CTRL+B HERE]')

Note: at the moment this works only for constant/hardcoded event names

Use Ctrl+Space), for example:
Mage::helper('[CTRL+SPACE HERE]')

This works with $this->_init([CTRL+SPACE HERE]) inside Models and Collections too

Use Ctrl+Space) with ->getTable calls, for example:

This works with $this->_init([CTRL+SPACE HERE]) inside ResourceModels too

Use Ctrl+Space) with Mage::getStoreConfig, for example:

Note: this works part by part, you probably need write a forward slash "/" and use Ctrl+Space again to get the final desired config path

Use Ctrl+Space with:
$this->getChild('Ctrl+Space')
$this->getChildHtml('Ctrl+Space')
$this->getBlock('Ctrl+Space')
$this->getBlockdHtml('Ctrl+Space')

Use Ctrl+Space inside any config.xml:

Tip: try to use it always, on any level, for nodes and also values.

Use Ctrl+Space inside any system.xml:

Tip: try to use it always, on any level, for nodes and also values.

Use Ctrl+Space inside any adminhtml.xml:

Tip: try to use it always, on any level, for nodes and also values.

Use Ctrl+Space inside any /layout/*.xml file:

Tip: try to use it always, on any level, for nodes, attributes and values.

Use Ctrl+Q when the cursor is over some node name or attribute name to get some brief documentation.

Note: at the moment not all the nodes and attributes are documented

Magicento will search rewrite conflicts automatically and will show you an alert if somo conflict is found

Use Alt+M when editing some .phtml and select the action Copy Template
Then you need to select the target Package and Theme (*), a new file with the same relative path will be created there.

* At the moment this only works for existent themes (so you need to create the folder first if you want to use a new Theme)

Use Alt+M when editing some file located in /code/local/Mage or some .phtml/.xml and select the action Compare with Original

Use Alt+M when editing any file and select the action Evaluate in Magento
Write the PHP code to evaluate (it will be executed inside your Magento so you can use Magento code)

By default the code is evaluated inside a "var_dump" call, that's why you don't need to put the semicolon, and you can evaluate just one sentence.
But this can be changed from Magicento Settings
When you are not using the "var_dump" option you can write any PHP code, but if you want to see some output you need to call "echo" (or some output function) by yourself.

Use Alt+M when editing any file and select the action Toggle Template Hints
This action will toggle the template hints for the frontend and also the backend in your current Magento

Use Alt+M when editing any file and select the action Fluch Cache
This action is very useful when you are developing with the cache enabled

Use Alt+M when editing any installer script (.php files inside /sql and /data folders) and select the action Remove this installer from core_resource table
This is useful when you want to run again some installer script
The core_resource table will be reseted to the prior script version so when you run Magento again the script will be executed automatically

Use Alt+M when the edit cursor is over some config path inside any Mage::getStoreConfig call and select the action Get Store Config
You can do the same evaluating the code with Alt+M > Evaluate in Magento
To obtain the specific value for some store, you need to set the current store from Magicento Settings (or use Alt+M > Set Store)

Select the text inside any .phtml file, use Alt+M and select the action Translate TExt

TIP: put the cursor in some part of the text to translate and use Ctrl+W to select the whole string quickly

Use Alt+M when the edit cursor is over some translated string and select the action Add Translation
Then type the translated text and select the translation .csv type (if you choose translate.csv and the file doesn't exist in the selected theme, it will be created)

Magicento includes the Magento dictionary from here: https://github.com/tim-bezhashvyly/phpstorm-magento-dictionary