For me it's the following:
3CX Droid Desktop - essentially a VNC server that can be accessed from any device on the same network
BMWhat - diagnostics software for most Beemers that allows reading of all ECU's in the car and resetting fault codes.
ConvertPad - Multiple formula conversion calculator
DTV India - Satellite TV for free
Electrodroid - Electrical/electronic calculator and pin-out diagrams
Fast File Transfer - Turns the phone in to a web server so data can be easily swapped with friends stupid enough to own a gayphone/gaypad
Hush SMS - An ethical hacking tool
PowerAmp - A "plays anything" audio player with some very nice features
RAR - Android version of WinRAR (compression/decompression utility)
SuperBackUp - An all in one utility that backs up SMS/MMS, Contacts, Call Logs, Calendars, Bookmarks, and all user installed apps making upgrading a handset a doddle
Text Bomber - Someone upset you?....buy a PAYG sim with free texts and spam the crap out of them
Torque - General OBDII diags tool for reading live engine data
Wi-Fi Inspect - Network sniffing utility
Wi-Fi Kill - Deny internet access to any device on the same network, useful in a pub on a Friday night when the bandwidth is pitiful with everyone connected at once
XBlast Tools - A utility that lets you modify your handset "on the fly" in pretty much any way you can think of, no more need to mess about with custom ROM's.
Simply mod your stock ROM how you want it and reboot. If you don't like what you have done, undo the mod, reboot, and you're back to a stock ROM again.