I have used Bonsai (
www.natara.com) to implement GTD (done right) on my Windows laptop and Palm PDA in a way that is as useful and perhaps more flexible than Omni Focus.
There is an interesting one called something like "Tudumo" -- very lightweight yet might be just what a person needs without undue complexity.
Right now my method of using Bonsai trumps all other options but I remain registered to Omni Focus in order to continue to evaluate its functionality.