Qiang has serious plans about keeping the core framework extremely minimal and implement optional, yet very useful components as official extensions.
I think he'd be happy if we listed all recommended libraries here. Many developers already use third-party classes (aka vendor libraries) in order to extend Yii's capabilities, and they are should be listed here as well.
At this time I can think of the following modules:
- Mail support (with SMTP, probably IMAP)
- XML-RPC (for trackbacks)
- Feed aggregating and generating
Let Yii spread. Good luck, Zii!