One of the tools unique to our Baseball Stathead site is the Neutralized Stats Finder. It allows you to search "neutralized" stat totals in order to compare players across seasons and in different run-environments. For example, you can find how many home runs Henry Aaron would have hit if he played all his games at Coors Field in 2000.
How do we do it? In a nutshell:
We adjust all of a player's seasons from the park and league context of the seasons they played in into either a "neutral" setting (which is 100 park factor with 162-game season, 90% of runs earned, and 688 runs/team), or into a setting selected by the user with a particular year, league (with its runs/game and earned runs percentage) and home team (with its park factor).
For more information and the nitty-gritty details on our neutralized and converted stats, check out this link.