I recently stumbled across frvade.com, and I'm almost wishing I hadn't. It's a series of increasingly difficult puzzles that are very different from anything I've seen before. It's inventive, addictive, and maddeningly frustrating, not to mention a serious time sink.
I got badly caught on level seven, but from there on half have been relatively easy and half have been very difficult. Ten and sixteen were rough. I'm now on level twenty and am trying desperately not to put too much time into it.