I don’t really have complex GTD needs. I just have todo’s and a calendar for the meetings/events I want to attend, so I don’t forget. iCal fits the last need very well, and I even take my calendar and todo list everywhere in my Windows Mobile Calendar thanks to SyncMate.
Anxiety is a OS X menu icon, that shows me the current ToDo, so I always get my work done. It’s really tiny and works across spaces (which I love). I really recommend people who don’t need advanced tools like iGTD or Things to try and work based on todo lists.
I do miss some features: Like changing the priority of items (or the due date, which is very useful for school assignments). And an option to show the top priority item in the menubar (like you can have the network you’re connected to).