When I was in Koln, I stayed at the Blacksheep hostel and it was pretty sweet. Definitely recommended.
Stayed in a 8 person room, it was very clean. They had a guy's bathroom, a girls' bathroom, and then an extra shower area that was co-ed.
There's free breakfast in the morning (just basic bread, jam, yada yada) and tea and stuff whenever you like. Internet is free too!
The owner is really nice too and knows a lot about the area, gives you a map right when you get there, will physically write on the map where some good bier gardens are and some good places to eat are and some cool things to see outside the normal stuff on Koln (ie, the Dom, etc).
VERY east to get to from the main train station. Get on like 16 or 18 of the U-bahn and it's like 5 stops away and a half block from the platform.
Definitely recommended to anyone staying in Koln! 20 Euros/night for a mixed dorm. Gotta pay 2 euros for your linen though, but if you're staying more than 2 nights, it's just 2 euros and that's it. If you stay only 2 nights it's 4 euros (doesn't make sense I know).