Get an open source, portable, future-proof, simple list: <a href="http://todotxt.com" rel="nofollow">http://todotxt.com</a><p>If you need a bit more complexity, keeping with open source tools, <a href="http://www.redmine.org" rel="nofollow">http://www.redmine.org</a> takes care of the job, easily. Plugins are a way to easily tweak it your way.<p>But falling for SaaS is easy, with <a href="http://trello.com" rel="nofollow">http://trello.com</a>, made by the guys at Fog Creek Software - which you might know already through the resounding name Joel Spolsky - is an awesome simple yet powerful tool, both for business and personal use.
The best way I've experienced is using a kanban board even for projects of one person. I use agilezen.com (free for 1-person projects) but there are a lot out there.