I use Remember The Milk (aka RTMilk or RTM) to keep track of the things I need to do and it really helps. RTM allows you to set up reminder lists that are easy to use and yet powerful enough to do more than just a checklist.
It is a free service with some additional PRO features available. The freemium model works really well with this as you can setup a group of people if you need to at no charge and pretty much do everything you need to do. The main PRO features are to allow you to use mobile devices (Android, Blackberry, iOS including iPad, Windows Mobile and Outlook integration.)
However, besides that you have full accessiblity via the web, email, Gmail, twitter, and even via an API for those that like to roll their own stuff.
Someone once joked that I was “Milking The Cow” when I was going through my lists.
Sign up for your own free account at http://www.RememberTheMilk.com
Check out the Tips and Tricks section for a bunch of ideas of what you can do with RTM.
I do have a PRO account that was given to me by RTM for a submission I made to their Tips and Tricks area. They have been giving them away every Tuesday as well as publishing the winning submission. All of the submissions are public in the Tips and Tricks area of the forum. I have previously paid for a PRO account and I have not received any compensation for this post nor for the register link above. I just really like the service and hope you do to.