There are myriads of online todo lists and "lite" project management sites.
Two of the nicer and well-known ones are http://voo2do.com
, both (currently) free.
But to me the lack of offline availability looks like a show-stopper for such an app.
I usually don't have online access when I'm on the road (maybe I could, but it would certainly be too pricey, regarding what mobile phone carriers and others charge here) and if your home or office network breaks down - like mine did when my DSL-switch-router-access point thingie at home suddenly died a few days ago and I couldn't afford a new one immediately - your're out in the cold. Not to mention the possibility of the maintainer of the site collapsing.
For Backpack, there's an offline Mac app (http://infinitenil.com/packrat/
) - which I haven't tried -, but nothing for the peecee so far. For the others, I haven't seen anything than hacked-together iCal integration.