This Japanese blog has some incredibly fun Java applets. It's mostly in Japanese, but on the right there's a handy list of 'games'. World of Sand (my personal favourite) also has a higher resolution version.
(Note: The navigation on the right is at the bottom of the page in Internet Explorer)
I think I've seen Hell of Sand before on another website. Didn't realize this was part of a whole series of games. Damn, these are mindlessly addicting.
Sweet little games. Too bad the site is all squigglies.