Cleaning stained carpet

We moved in to our 1956 Eichler in 1996. At the time it had off-white carpeting that even then had seen better days. As my wife was about to deliver twins, however, we say no point in replacing just in time for real mess-makers to arrive.

Well, now the kids are six, and the formerly off-white carpet is WAY off-white--with almost black splotches around the kids' seats at the dinner table. We really should replace the whole thing, but we're planning to remodel in a few years, and would rather not do everything backwards.

Can anyone recommend any carpet cleaning companies that can do serious spot-removal?