Automomatic typographical error correction
I will reference two sources:


Wikipedia has its own lists: Lists of common misspellings
Aspell is certainly a place to visit. You can find a list of misspellings and corrections here.


Still, it is a good idea to think of using a spell checker, such as Aspell, since it gives you the ability to deal with no-so-common misspellings (of course, at the cost of false positives). A good starting point is Peter Norvig's essay on creating a spelling corrector.