Jay Is Games is the best source.
Desktop Tower Defense - Pure crack.
Vector Tower Defense - Freebasing.
PicCross - I've always loved the concept behind these puzzles, but found them to be tedious on paper.
And that's just the three that I play the most.
Crazy addictive. Unfortunately, many of these games force you to play in a browser window surrounded by ads. If you know what you're doing, you can go into the source code and find the server link for the game SWF file, download it directly, and then open it as a file in your browser. Hello, full-screen DTD! Of course, they disabled that ability with the 1.5 version, so I still play 1.2. Oh well.
Oh, lest I forget, all "escape the room" games. Some of them require a ridiculous amount of pixel hunting, which takes some of the enjoyment out of it when you're more worried about the accuracy of your mouse than your logic skills, but still, great fun.
Sphere was my favorite.
UPDATE: As it happens, Slate had an article about this a couple weeks ago. Many good choices there.
