PBS dev. tracker cracks me up, but it is good for general use. It said that a 2 year old will not do X yet (sometimes about alphabet or language), but dd did it before 2. I just think it is odd that they said "will not," like there are no possible variotions from "normal" also babycenter.com has a decent milestones tracker for babies and kids.
It doesn't crack me up but I sometimes think it can't possibly be accurate for the same reasons you listed!!!! Yet I hear it's pretty good....