One of my favorite to-do list applications is Tasque, made by the same folks who created the very cool note-taking app Tomboy. Yesterday the developers released 0.1.6, that fixes some bugs and adds a couple of new features, including a new task entry widget at the top of the app's window.
Tasque is a lightweight little app that doesn't have a ton of features yet (but it's one of the few that integrates with the popular Web-based reminder app Remember The Milk), so if you're looking for something more robust, there are plenty of options out there. Let's take a look at a few.
If you're using Kontact or Evolution as your personal information manager (PIM) then you already have to-do list functionality at your fingertips. This is a particularly useful way to go if you use either of these PIMs since to-dos are already neatly integrated with your email, calendar, contacts, and notes.
Looking for a tiny and simple app? Try iKog. It's text-only, there's no GUI, but it does give a nod to the Getting Things Done crowd by allowing you to add contexts and priorities. The best part about iKog is that it's cross-platform and small enough to be carried on a thumb drive so you can take it (and your to-do lists) everywhere with you.
For something with fuller features, consider Task Coach. Set tasks to alert or reoccur, sort by attribute, view by list or as a tree, and more. You can also drag and drop emails from Thunderbird to create a new task, and add files, emails, and URLs as attachments. Like iKog, Task Coach is small enough to be transported on a thumb drive.
If you really want to get back to basics, keep your to-do list in a plain text file. If you go this route, have a look at Todo.txt, a set of helper scripts that "slice, dice, sort, distribute, pipe and munge your lists any way you please."