Ad-away over AdFree (Ad-Away is opensource, adfree is not, and had some issues with adree's dev not releasing release notes etc).
WIFI Teather (Use the beta version from the site, and not the release ver. from market).
EZExplorer (has a SU option, and mount /system in rw/ro options).
Volume+ (Free on XDA, paid in market) - really good to increase the speaker volume past the stock limit
AndroSS - open source screen shots application
ChainFire 3D + Pro (if you want to play tegra games). Trust me it is really worth paying for this
CF.lumen - Great for reducing eye strain (you need to buy ChainFire 3d pro first)
CPU Tuner - open source CPU over/under clocking.
Quick Boot - reboot to recovery from a shotcut on home screen
Script manager - For running scripts (superchargerv6 et. al.)
Teather GPS - used in conjunction with wifi teather, to get GPS navigation on my 7" nook color
TB Pro - You always need Titanium Backup
That should get you started.
One thing I would suggest when searching root apps, is search if there is an open source alternative. Don't like giving root access to closed source apps. Quite a few apps I have mentioned are indeed open source.
All the best