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You could use Trailer Park;-)

Joking aside, depending on what you're doing, you could probably get by with something as simple as a Google spreadsheet.

I want built in notifications, comments and task completion. I could get this via Google docs, but it isn't seamless.

you can setup a http://redmine.org instance . I've done it recently for my company and it hasn't broke yet :) if you n€€d help let me know :)