Mr Driller is the most addictive puzzle game I have played in a long time. Also Super Puzzle Fighter II X is very good.
I played Plumb Plus ages ago but didn't think it was that good.
The only other one I can think of is The Next Tetris Online, but alas it is a failrly lacklustre version of the classic game.
I hope this helps 🙂
(Edited by Eleven of 9 at 9:05 am on Nov. 21, 2001)