Here's a list of some CTF practice sites and tools or CTFs that are long-running. Thanks, RSnake for starting the original that this is based on. If you have any corrections or suggestions, feel free to email ctf at the domain psifertex with a dot com tld.
Whether they're being updated, contain high quality challenges, or just have a lot of depth, these are probably where you want to spend the most time.
Just around for historical sake, or on the off-chance they come back.