i would say start here
General - Registry Hacks, Tweaks and .cabs
http://forum.ppcgeeks.com/showthread.php?t=68241
If you're looking for apps that allow you to spice up your today screen, you can check out the following:
Ultimate Launcher
SPB Mobile Shell3
PointUI
Wisbar Advanced Desktop
Can't think of any other ones right now. Hopefully that helps to get you started