I would recommend Mxtube if you use youtube a lot. It allows you to download the youtube videos right to the phone over wifi or edge. It is under the bigboss source which is already on the Installer if you installed Community Sources.
For IM programs, I personally prefer mobilechat over Apollo. That is also with community sources.
Summerboard is good for changing the homescreen theme. Plenty of themes to download also in the installer. Also under community sources.
If you like watching and listening to video and audio podcasts, you can download MobileCast so you can get the podcasts without having a computer. Also through community sources.
If you use IRC chat, download colloQ also under community sources