Two task / light project management apps I can recommend:
Free:
Orchestra.com -- this is still being enhanced and lacks some basics such as search and tags. But it's a pretty good early effort that allows you to assign tasks to other people via email pretty seamlessly. This is the first shared task manager that I haven't had someone come back and say ""now how do I work this"" when they've been assigned a task in testing.
Paid ($99/yr):
Flow (getflow.com) -- this is an ""all grown up"" version of Orchestra that is about a year ahead development wise. The biggest downside being that there's an annual fee of $99 (30% discount for 3 or more -- 50% for 10 or more).