There are some great videos around to give you an idea about what you can do with Asterisk and FreePBX.
Here are a selection –
Kerry Garrison, the senior product manager of Trixbox gives a quick tour of the installation and setup of Trixbox 2.2. The first half of this video concentrates on installing Trixbox but if you have a Trixbox VPS the hard work is done for you. Trixbox is now on version 2.4
Trixbox features. A nice run through of some of the features in FreePBX/Trixbox.
A good (and pretty long!) explanation of what you can do with Asterisk. This doesn’t include any information about FreePBX, the web based GUI.