I have an X10 and on it have:
Barcode Scanner - Scan bar-codes on things and find out the price through google, really useful for students to find the price of books etc
Contact2Sim - Copies all contacts too and from the sim to back them up, useful when you first get the phone and all contacts are on the sim card.
Draw - For pissing about with, just like paint but touch screen.
Formulas Lite - Just a load of algebraic formulae and the rearrangements of them (Useful for me doing engineering)
*Notepad* - Free p**n app that disguises itself as a notepad.
OI File Manager - For managing files, also makes it easy to open pics/videos and documents etc.
Periodic Table - Just what it says, but loads of other sciency stuff too.
Robo Defense - Dangerously addictive game
Smart Alarm Clock - App that monitors your sleep patterns to that you wake up at the end of a sleep cycle not in the middle of one.
Task Killer - Kill unused apps etc
The Free Dictionary - Used for finding out what abbreviations mean mainly, but is very useful.
Trapster - Yet to actually use, but apparently it is a network of users all relaying information to each other regarding police locations, speed camera, traffic jams etc.
I'm an engineer and its exam time
will get some more 'fun' apps when I have some spare time