TCPMP/Coreplayer (Video/Music/Media Player) and Google Maps are among the top two programs I'd install no matter what. PocketShield (
hxxp://pocketshield.netserver01.com/) is a commercial program, but a fantastic slide to unlock type program that works very unintrusively, if that's a word. Those 3 programs I'd recommend to anyone, no matter what their phone habits are.
WMWifiRouter is a nifty program too, that lets you broadcast your 3G data over wireless and can let your laptop get wireless data on the go.
If you have the stock Sprint ROM, you should look into the Sprint cleanup cab as well. It made my stock ROM way snappy and was on-par with the faster custom ROMs.