I love my babysitting co-op! It's such a lifesaver. We have a weekly playgroup where the kids play and snack and we talk and snack. If we need a sit, we drop our kid(s) off at a member's house and they play together without even knowing it's a "sit." They think it's just a playdate and don't care about what mom and dad are doing at that point because they are with kids and moms they know well.
I started our group nearly 3 years ago. I used the Smart Mom's Baby-Sitting Co-Op Handbook: How We Solved the Baby-Sitter Puzzle by Gary Meyers as a guide. Right now one of the co-op kids is here playing with my daughter for 3 hours while I clean, do laundry, play on the computer. PHew! It gives me a break to do a sit for another 4 year old. Not only that, but my Dh and I have a date set for this weekend and our daughter will be at a playdate while we do it.
I feel like this could help other SAHMs, but no one ever seems interested.