The guy who created activeCollab first released it as open source → http://web.archive.org/web/20060810011430/http://www.activec...

Then, he realized it could make money and put a $499 price tag on it → https://www.activecollab.com/pricing.html

I'm open to code the frontend. I just need one guy or two who can do the backend side.

I've never used Basecamp, but... Teambox? Last version seems SaaS only, but previous ones are OSS:

http://teambox.com/ https://github.com/teambox/teambox