Google recently added Tasks via a Gmail Labs add-on. I find this simple task list to be quite useful. However, it's tied to Gmail in that Gmail has to be open in a browser window for it to function. The "pop-out" feature is nice, but the Tasks lists automatically closes when the main Gmail window is closed.
Shortly after the initial launch, Google announced a version of Tasks optimized for mobile clients. If I had an iPhone, that'd be great, but I don't. However, I was able to exploit this to make a Gmail Task application!
I used Fluid to run the Gmail Tasks designed for the iPhone to get a simple task list that doesn't require you to keep Gmail open. Gmail Tasks for iPhone is only displayed for mobile broswers, so you'll need to swap the User Agent string in Fluid to make this work.
Create a Fluid App pointing to http://mail.google.com/tasks (Google Apps users should use http://mail.google.com/tasks/a/YOURDOMAIN). The first start-up will fail, because you need to swap the User Agent to Mobile Safari. In your Fluid app, under the application menu, choose User Agent and select Other... and put in the User Agent String for the current Mobile Safari:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090303112902476