Task Coach is an absolutely fantastic to-do manager that's very easy to use, but has plenty of wonderful features. The program is released under the GPL, read more about it from the Official Site.
I highly suggest you try this out as it's a wonderful program that's beneficial to many an goes great alongside PNotes (Portable). If you do download it, please do me the favor of reporting feedback on any problems you have, or even typos.
Update - Task Coach 0.66.3 (Bug Fix) was release today 12/12/07
An update is coming soon. I'm just working on adding in all the avail. languages into the installer. Everything else is good to go.