Ask HN: What software do you wish had a good open source alternative?

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Disclosure: My professor at CMU runs the company.

Anki - https://github.com/dae/anki

It's obviously not as full featured as Rosetta Stone, nor does it come with courses, nor does it use voice recognition - but it does rely on basically the same research into memory and I've had a lot more success using Anki than Rosetta Stone.

Groupware is still the big unsolved small-business problem in an open source stack. There have been many, many attempts to fix individual pieces of the problem, and a handful of overarching ones, but I'm not sure anyone has nailed it.

Client side: Evolution kinda/sorta works, most of the time. (And it doesn't run well on Windows or OSX.) The old Mozilla suite has just exploded -- it now takes three (haphazardly maintained -- versions last a few months at most) applications to manage what it used to do.

Server side: open source mail and mailbox servers are among the best, but is there any consensus on the best calendaring ? What clients can even talk to a CALdav server? Outlook can't.