I've heard that some discount key sites sell stolen keys.  I've used greenmangaming.com, which is legit as far as I can tell, but they don't have some of the titles I'm looking for.

Are there any other legit sites with a good selection of Steam keys at a significant discount?

I've bought from cdkeys.com before and never had any issues

I used G2A to buy a key for Prey (the old one that is not available anywhere) last month. It worked perfect, although I doubted it would be legit.

All your questions are now answered Different CD-key Retailers

cdkeys is legit from their disclaimer.

Guessing at some point they will end up like Greenman and prices will rise.

gamesplanet

Just be sure to check the page to confirm that the key isn't Europe region locked. Most games aren't

Since they are legit, don't expect a huuuuge discount. Usually I save like $5-10 off the steam price. Some launch games end up being around $15-20 off.

Thanks for all the responses so far.  I decided to buy GTA V for $30 from cdkeys.  It's not a Steam key; I had to activate it on Rockstar Social Club.  But for $30 savings over the Steam price, I figured it was worth the trouble.

They have a couple of other titles I'm interested in at good prices, but I think GTA V should keep me busy for a while.  Also, I just bought another 500 GB SSD drive to put games on and it's now 80% full.  

I'll check out the other sites that were suggested soon.  

You know the steam summer sale starts in a few weeks.