Well, new for me at least…
The first one is 0x41414141.com. Just go to the site and you can start directly. As far as I know, this is not time-bound.
The second one is spargecoduasta.com (“break this code”). It is put up by BitDefender and I don’t know if it has a time limit. The levels I’ve seen seem to focus on C/C++. It is available in both Romanian and English.
Finally, a little off-topic, but still a challenge: The Science Knowledge Quiz – with the tagline “Are you more science-savvy than the average American?”. Via Pat’s Daily Grind (I’ve got 11 out of the 12).