I'm part of the Twin Cities East chapter of Mothers & More (www.mothersandmoretce.org) which someone already mentioned. Among other things, part of what I really enjoy is that the mothers who belong are more interested in what we have in common vs. whether or not we're at-home parents.
There's a good blend of at-home parents, people making work/home transitions like the one you are making, and people who are juggling paid work and parenting. The other things I like are that I won't suddenly "outgrow" the organization because my kids reach a certain age, and that we have a mix of daytime and evening activities.
We have a chapter meeting on Tues., 1/22 called "Healthy Home: Limiting Family Exposure to Toxic Substances" all about being more aware of the toxins in common household products our families use. Details are at the website -- come check it out if you're available!