Like someone else mentioned, clear out the drain filter. Also make sure your drain tube (not inside the machine, but the tubing that actually drains into your house pipes) is not blocked. Unfortunately it's possible for socks to get stuck in there! And we all thought it was the dryer that eats socks... :)
If that doesn't do it, you can do the hot water cycle with just bleach and/or vinegar-which you should do occasionally anyway to clear things out. If the smell gets really bad, I have found that Whirl-Out is the best thing to clear out all of the pipes, etc. It is what was suggested to us by our manufacturer when we first got that smell a year or two after we switched to the front load machines. It is a whirlpool cleaner that you can find at Lowes or Home Depot (you have to ask, and make them check their system even if they've never heard of it. They will almost always have it and the clerks there have no idea about it), or you can order it online.
We have had ours for nearly 5 years, and with 7 kids (and 2 dogs) ours gets entirely too much use-usually around 20 loads/week. Between regularly checking the drain filter (use a pitcher like someone mentioned-there is a LOT of water in there!), and the clean out wash cycle, with using Whirl Out once a year, as well as a good wipe down of the rubber seals around the outside every week, it's running good as new, and odors have been kept to a minimum!