OpenERP is pretty damn feature complete
I'll second this. We used this to replace Exchange in house years ago, and it runs great. We've got 350 users on it right now running Outlook / Webmail / ActiveSync / BES / etc, with the system still running nice and fast.
Just replace the heatsink on an AMD with the frozen turkey. In about 15 minutes you'll have a fully cooked turkey.
So far I've set up several small offices using Samba4 as a drop in replacement for Active Directory. Here is what I've found it does well: Windows Authentication, AD DNS, Group Policy, Easy scripting (python tools and libraries). What it doesn't do well yet: Replicating AD with other servers. I haven't had much experience using subdomains, etc, mainly because I haven't been able to get it to replicate. But for a small office, it works fine.
My company uses FormFlow Filler extensively, for filling out government forms. It was bought by Adobe and killed, with the end of life in 2004. It barely runs on XP, let alone anything later. If it was open sourced, I'm sure even with my meager coding skills I could at least keep it functional, since it is still heavily used. Point being, proprietary software being shelved sucks much more than open source software being shelved.