Also, some of the playgroups/community centres let you hire them out. I know Moana does and so does the old cottage playgroup. You just have to supply the food/drinks, and they get free run of the playgroup, i.e. toys, slides, cubbyhouse, climbing frames etc.
Haven't actually done this one myself, but know a friend who has, and it worked well. You still have to do the cleaning afterwards, but hey, at least your house isn't in a mess.