We all know who Atari is... I hope :) They were ready to jump into the market with several handheld games (and some larger tabletop games which look kind of like electronic board games). Due to one circumstance or another, the only handheld game they actually produced was Touch Me. Everything else on this list is prototypes (some working) that were never actually sold.
Cosmos
Pro Ball
Pro Darts
Space Invaders
Super Breakout
Touch Me
Tronic 2