In the April 2011 issue of Popular Science there is a one-page article about changing the Android system. Some of the things mentioned:
Go to xda-developers.com on how to root or ROMs (it also suggested using the app ROM Manager)
1- Improve the LOOK
A- Replace the Home Screen with either SlideScreen or LauncherPro.
B- Kill the ads with AdFree (rooting required).
2-Improve the FEATURES
A- Steam your media using Twonky
B- Share your connection with either PdaNet or Wifi Tether (rooting required).
3- Improve the PERFORMANCE
A- Save power with either JuiceDefender or UltimateJuice.
B- Boost speed with SetCPU (rooting required).
Not too bad, although when I looked at SlideScreen it was hard to find a good comment.