I tried Basecamp and some iOS todo list tools, but in the end, I have one Google Doc with lists of tasks. I should dump Google for Evernote (better mobile support and less connection issues) but I once had a sync issue on Evernote and lost some text, so I don't trust it. I might go with just a text file in Dropbox or use Gina's http://todotxt.com/
I keep shuffling how I organize tasks. There's by-project, where I list all the things that need to be done to complete something. But I also have to keep some context lists like "next time I'm working on the website" or "next time I feel like doing a bunch of Photoshop work" for those tasks that keep getting put off.
I also keep a wall calendar (just a big sheet of white paper) with post-its for each deliverable. They keep slipping, but at least I can see how many weeks are left until external events.
When things get really unproductive, I walk away from the computer and sit down with a notebook or stack of index cards/post-its and dump everything that needs to be done. For tasks that are stuck, I try to apply Merlin's idea of visualizing what it will feel like to have it done: http://www.43folders.com/2005/10/16/43f-podcast-the-to-have-...