If you enjoy jigsaw puzzles, take a look at www.thejigsawpuzzles.com. It's an online collection of Flash-based puzzles, ready to solve simply by dragging with your PC's mouse.
Unlike tradtional box-based puzzles, these online versions have a number of advantages. Having chosen your picture from the thousands on offer, you can then choose how many pieces you want. In the image below I've opted for 20, but you can choose 2000 if you're feeling brave! You can save part-completed puzzles to return to later, and there's never any chance of there being a piece missing!
All in all, it's a great online puzzle experience with nothing to download and install. It'll run in any browser that supports Flash (so apologies to iPad users).