Vector Tanks - great battlezone clone, spot on. Adventure - from the 2600, free and very cool. Space Invaders Infinity - very cool take on the old classic. Rogue Touch - great 2d rogue game from the atari 1040 days. Right off the top, those are probably some of my favorite 'retro' type games.
Wolf 3D. Adventure (if it's still available). Q*Bert. Pandemonium (depending on how old you mean by retro). Those are just off the top of my head...
yeah I've got Adventure it's pretty cool (for free) other favorites I've got are Flashback Tyrian Myst Wolfenstein
Wild West Pinball? He said he is looking for actual retro games, not new games with a retro feel to them...