You can download the free, 30-day trial version of After Effects here.
The trial version of After Effects CS5 and earlier had a major shortcoming: It lacked many software components that we licensed from other companies, including many codecs for importing and exporting common formats, such as MPEG-2, AVCHD, and RED (R3D) media. The same applied to plug-ins and utilities that we license and include with After Effects. This meant that people had a hard time evaluating the software for real-world use.
Things have changed for the current version of After Effects, making the trial version more useful for its intended purpose—letting you try the software as you will really use it.
The trial version of After Effects includes all of the codecs and effect plug-ins that are included with the full version of After Effects. This means that you can import and export to all of the supported file formats using the trial version.
The trial version of After Effects also includes the Keylight plug-in, mocha-AE, mocha shape, Cycore (CC) effects, and Color Finesse. (Hooray!)
For lists of formats that you can import into and export from After Effects, see these pages: