I've used ResultsManager (based on Mindmanager), the GTD Add-in for Outlook, Kinkless GTD (Mac software , needs OmniOutliner), and various other hacks (text files with AutoHotKey macros, MS Excel spreadsheet, and Outlook). There's one called "My Life Organized" that I believe is based on GTD. (I don't have the links to these tools, but you shld be able to Google them out).
My tool of choice is Zoot (
http://www.zootsoftware.com). Its feature set is best summarized as "Outlook on steroids", but the main reason I use it is because it lets me GTD *faster*. Its design follows the "capture first organize later" paradigm that makes it incredibly unintrusive to work with. Highly recommended, for GTD and more.