I really love Tadalist, which lets me keep a to-do list online. A few annoyances are fixed by installing Rich Manalang's Ta-da List Enhancements Greasemonkey script. It adds all of your lists to the sidebar, so you can switch back and forth easily, which was probably the major problem with the web app
Another thing that always annoyed me was that all of your completed items appeared at the bottom of your list. Sure, they were greyed out and smaller, but I like to forget about things once they're completed. So, I tweaked the script to hide all of those annoying reminders of past duties. In the future, I may add a box to toggle this ability, but for now, this suits my purposes just fine.
The script is released under the terms of the GPL, and is available here_ Ta-da List Enhancements Enhancement