I'm still a fan of the old-fashioned stuff: solitaire
(I like the cards bouncing around a la desktop Windows
); Tetris
(Peter's GameBox is cool for this & some extra games
); Wordz
(Boggle-style letter-grid game
); Invaders
(almost an exact clone - thanx, Scott Ludwig
); & Same Game. I've also gotten hold of WizardWorks' Casino Pack
(US $6 on eBay, including S & H
), which is a pretty good emulation of Vegas casino games - the graphics are good & the rules are consistent with Vegas.
(I was getting tired of not being able to double down and/or surrender in other CE blackjack games.
) I'm also considering H/PC versions of Monopoly & Scrabble that are still out there.
Honestly, though, I've been somewhat disappointed at the dearth of freeware games. Palm OS is still way ahead in this department.
(Sure, the games aren't as sophisticated, but sometimes all you want is the game, not fancy effects.
)
(Except for the cards in Freecell and Blackjack, which are the same graphics as the cards in Solitaire, and so are in color. Go figure.) Still amazes me how many hours I can waste playing Minesweeper some times... 
