In a rare event of tada posting a link in IRC that wasn't disgusting, unfunny, something I'd seen before, or a combination of the three, I discovered Mamono Sweeper. My first suggestion would be that if you're someone who enjoys puzzles, doesn't give up easily and/or finds it hard to resist difficult challenges laid before them, then don't click that link if you're planning on doing anything else for the rest of the day.
It's essentially a minesweeper variant, with an RPG element sort of added in, but not really. Instead of mines, you have monsters of various levels. Click on a level equal or lower than yours and you safely kill it. Click on one stronger than you, and you lose health. You're given a small amount of health to start with for gambles, mistakes and I guess tactical losses, but it doesn't last very long, and clicking on anything more than a level or two above you will kill you instantly anyway. As you kill monsters you gain experience and level up. Usually to level three or four (out of nine) before you fuck up somewhere and die.
It actually comes out as a cross between regular minesweeper and Kakuro puzzles. Instead of the number of monsters around each square, the number on regular squares indicates the sum of the levels of all the monsters in the squares adjacent to it. It means that playing quickly requires some mean arithmetic, and you have to be fairly accurate with it unless you want to lose a lot. It's pretty fun, but also really difficult on a board that size, and it's addictive as hell.
I spent fucking ages over the weekend trying to beat it, and I still haven't managed it. I did a couple of the easier modes, but the hardest one still eludes me. I'll get pretty far (usually well into level three or four) and then fuck up and usually mis-click a square by mistake, or occasionally I'll make a mistake with adding and die as a result.
Oh, and if you click on the squares of monsters you've killed, it toggles the number for that square. Which is fucking useful, and I only discovered it after playing for two hours first.
Also, today's Dilbert comic was pretty funny: