Which city do you live in? I am part of several groups that meet alot. One group is MOMS Club which are specific to the area you live in and the age of your children.... but the plus side to that is that everyone lives close, so can meet up easily/more frequently, your kids can develop long time relationships and the kids will be of similar developmental stages. I think you would also be part of the Little Brothers/Little Sisters that meet an additional day a week...it's for moms with more than one child so kids are all different various ages. We meet at parks, swimming places, indoor play places, peoples homes, the zoo, etc I really enjoy it, but admit that I too was a little turned off at first by the 'membership'... it's only $20 a year and it covers some costs of things the group does to support moms during the year. It's been more than worth it for me and my daughter has had a blast and made so many friends. Once you have made friends, lots of people meet more often than the scheduled playdates, dinners, moms night out, parties, etc just for their own playdates...
Also - are you nursing? If so, I can recommend another GREAT group of moms that meet for outings and playdates as well....
Hope to see you at something soon :-)