I tried Nozbe and I liked it. I also tried ToodleDo, and I liked it even more. These are probably the top two to-do list apps our there.
Update: I have now largely abandoned ToodleDo in favor of Workflowy (also recommended here), and use it pretty much like Paul Christiano does (as described in My Workflow).
I have often benefited from recommendations for Things I Didn't Know I Wanted.
Given that Less Wrong is a community of unusually intelligent, critical, and self-improvement-focused people, I suspect we can generate a pretty helpful thread of product recommendations — perhaps even a monthly thread of product recommendations.
Rules: