Ones I use:
Games:
Bubble Blast 2 - simple puzzle game
Angry Birds -
Gameboid Light - almost all GBA games on your phone no explanation needed
Fruit Slice - free rip off of the Fruit Ninja game
Speedx - uses accelerometer to avoid obstacles. pretty addicting
Apps:
XDA Developers - if you browse XDA forums then this is a must
Album Art Grabber - great if your stock player doesn't get the album art when you transfer music
Minimilist Text - great to make your homescreen look awesome
Circle Battery Widget - 1% increments, looks great too
DroidLight - flashlight dev by Motorola
Google Sky Maps - if your into astronomy then get this
GTasks - syncs with gmail account and displays reminders also can add quick note/todo lists. its really simple to use and looks great too
Maverick - Best offline GPS. preload maps while online and it stores the tiles or you can dl maps
Barcode Scanner - pretty much self explainatory. best one on the marketplace imo.
SiMi Clock Widget - awesome date and time widget
Blue Skies Live Wallpaper -the best LW on the marketplace right now. looks amazing!!!