Project Management Software

I need an online to-do list, but one with subtasks that will comprehend enough complexity for my needs.  Maybe something on sourceforge, or even just a google app .  We used to use bugtracker.net at my old company, but that’s in asp.net and I while I’m using IIS I’m still mainly php based (and don’t know enough to switch willy-nilly.)  And I really wish there were a simple answer between whether I need a project management program, a collaboration