Ask HN: A legitimate, open-source, Sharepoint alternative?

HN,

I work for a midsize start-up (~30 employees). We've been using a very old version of Sharepoint and it's time to kick it to the curb. It's nearly unusable (technical issues) and it's basically a glorified file server.

I've done the research on the costs of migrating to a newer version, and while we can eat the cost, it's still a significant sum (SQL Server/Sharepoint CALs).

I'm a Linux guy and I would prefer to use something FOSS. There's no question that Sharepoint is an incredibly powerful tool, and it's probably worth the cost. With that said, I know there's something good out there.

Throw some knowledge over here if you know of any true alternatives to Sharepoint. Bonus points if simplicity is a factor.

Thanks, SoMuchToGrok