The tip of the iceberg.
The public open source repository "SourceForge" lists around three hundred VOIP projects.  One of the better known of these is Asterisk@Home, which was originally designed for home users and provides a web-based simplification layer in front of Asterisk.  At the other end is Asterisk Business Edition which is commercially tested and supported.
Web management:
FreePBX, ACaMI, Callware VoiceOne, AstWeb - Asterisk PBX Web Interface, Asterisk Realtime Web Configuration, Ampersand
Full systems:
ST-PBX Live, Asterisk@Home, MyAsterisk
Enhancers:
app-conference, Enhance Me, AsterOID, Ruby Asterisk Gateway Interface
Billing:
*starShop, Astbill, A2billing
Sounds
Want more/better/different sounds files? Get extra-sounds from Asterisk itself, ora"native sounds" which are recorded in different formats requiring less compression and providing better sound quality. Or choose from several commercial sounds suppliers.