As we said earlier, there are many to-do list options and services online and you can find one to meet your needs. Here is another good one: Remember the Milk.
Remember the Milk is really flexible, free, and nice to look at. It is organized using tabs (like file folders) and you can even make smart lists (lists culled from other lists that you set up by specifying how/why something belongs on that list). You can even use RTM offline and then sync when you go back online.
It's social too- you can share tasks with friends or colleagues.
The down side is that the mobile app will cost you an upgrade to the "Pro" account online, but the cost isn't too much ($20/year). Your mobile app can then be synced with the online version and vice versa. If you are looking to get organized and this interface appeals to you, give it a try. Remember the Milk has a lot of fans!