Try looking at meetup.com or Yahoo groups.
Here are 3 wonderful groups I belong to that have playgroups. Some of them take place at homes and sometimes we meet at parks, pools, mall play areas, etc.
http://groups.yahoo.com/group/ElginAPgroup/
(This is a new group of moms in Elgin and surrounding areas. The focus of the group is AP or Attachment Parenting. If you don't know what AP consists of, it is basically meeting the needs of your child through your instincts, such as sleeping with him/her, breastfeeding, wearing your child in a carrier or sling, and gentle discipline.)
http://mommymilk.meetup.com/373/
(This group is for moms who either are breastfeeding or have breastfed their little ones. It is local to Elgin/Algonquin and that general area, so it is a closeknit group. It is also a fairly new group. We have had meetings at La Leche League, the farmer's market, the library play area, and are going to a park today.)
http://alternaparent.meetup.com/86/
(This group is for alternative parents. This includes AP parenting, parenting with values that may not be mainstream, same-gender parents, etc etc... It is spread throughout the Chicagoland area and not all events are held near Elgin/Algonquin. There are small playgroup events planned as well as larger events such as zoos, museums, etc. The ages of children in this group tend to range a bit more from baby to school-aged.)
Hope this helps! Feel free to send me a message if you have questions about joining any of these!
Amanda