Here are all the cash back sites I've found for Groupon.
What you do is go to the site you want to use, enter Groupon as the store, click their shopping link, then buy away at Groupon like you normally would. You DO have to do this every time you do a separate transaction in order to get credit.
And without further adieu, here is the list:
20% cash back. ShopDiscover (for Discover Card holders only)
8% cash back. MrRebates (anyone can use AND you get a sign-up bonus!)
6% cash back. Fat Wallet (open to anyone and also has forums, coupons, and other features, as well)
3% cash back. Ebates (anyone can use Ebates AND you get $5 just for signing up!)
If you know of any others, be sure to post a comment, and I'll add it to the list!