I like this one http://thejigsawpuzzles.com

I use Magic Puzzles.

I use jidigi. You can save your puzzles and go back to finish later. You do need to sign up but only so you can bookmark ones you want to do and save the ones you are working on. I will have to check out the others, but this one I really like.

I use http://www.jspuzzles.com/

And there is this one...

http://www.jigidi.com/

I use jigidi. I tend to do the 400 piece puzzles and it takes me more than an hour to do (not usually at one time). If you register and sign in your puzzles are saved and the timer stops so you can go back. I don't understand how some people can do the same puzzle in 20 minutes that takes me an hour to do haha I do it for enjoyment not the race to get it done. I really like the geometric puzzles instead of scenes.