The games I have on my Droid right now are (all can be downloaded for free, if not in a lite version that has plenty of gameplay):
Gameboid - plays GBA roms perfectly, MUST HAVE, play pokemon emerald and mario kart on this
SNesoid - plays SNES games such as mario, MUST HAVE
Robo Defense - tower defense game, extremely addicting, great gameplay, must have
Abduction! - simple game where you are a cow jumping up platforms towards an alien ship, controlled by accelerometer, addicting, passes time fairly well
Labyrinth - control a ball by tilting the phone and direct it though a maze into a hole at the end, everyone will want to play this on your phone
Jewels - get three jewels in a row to eliminate them from the board, very addicting
Pong Multi Touch - just like it says, utilizes the multi-touch feature, works well with a friend
Phit Extreme - very hard puzzle game, fit tetris-like pieces into a rectangular space
Solitaire - speaks for itself