You should definitely check out the Tablesmith and Campaign Sage that was mentioned.
Both are free and I think the Tablesmith is a godsend.
In addition, I have recently tried and tested the others listed above, and absolutely NONE match the quick response you get on ALL queries about Tablesmith (which, as mentioned, is free) from the designer (it's amazing how helpful he is since it is free.)
In my continuing testing, I did discover DM Genie which is really surprizing me thus far. It costs money, like some of the others listed above, but is not in as much of a beta stage as they appear to be (I need to put a disclaimer here....this is the last in a long series of tests that I have been running and I have not truly explored any of them FULLY, and DM Genie I have explored the least....but, it has a 60 day test period and I'm still going at it=)
Also, as a note, I did find PC Gen useful, but it uses Java which I find to be the slowest engine of them all....perhaps this is more of a testament to my lack of patience, but it's my opinion anyway=)