I'm a big fan of gootodo.com, but I wanted an easier way to add quick todo items, so I hacked together a greasemonkey script for Firefox that adds a "Quick Add" text box below the todo list. Here's a screen shot, and here's the script. (If you have Greasemonkey installed, just right-click the link and select "Install User Script." You'll probably need to add my site as a trusted source.)
Enjoy!