Tim Gaden at Hawk Wings has compiled a list of ten Mac OS X tools that can help you implement the Getting Things Done philosophy in the way you work. The tools are broken down into three categories: email clients, dedicated GTD apps and web-based solutions for that ultimate cross-platform effect.
Mail.app, of course, starts off the list. Once combined with a few Smart Folders and the powerful Mail Act-on and MailTags plugins, Mail.app can become a most powerful GTD tool indeed. Other featured tools include kGTD (an OmniOutliner Pro template), a Todo.txt script for the ultimate in plain-text GTD, EasyTask Manaager (a stand-alone app without the GTD learning curve), iCommit (a web-based PHP tool), Backpack, Gmail and even wikis.
Check out Hawk Wings' list for more ideas and tools to help you, well, get things done.