Interesting thread. I'm surprise no one has mention https://jitsi.org/.

plus:
- work on windows, linux and macs
- can do it all in one cliet (voip, dktp sharing, im (with multi serv's),
- has settings for video quality control amoung other advance settings
- is completely open source
- I was pleased with the quality (more or less )
- you can set up a free contact to video/call/im from.
- you can set up a home serer to do all this through if you want complete control
- easy to set up (using jitsi xmpp service)
- I'm sure a few more I'm forgetting

Minus:
- uses Java as it's main platform.
- Interface design is okay feels a little dated.
- jitsi provide contact can't be reach by regular phones.
- no maemo native version.
- a few others I may be forgetting

Overall, closest thing to a skype replacement I've seen so far. Have a look I curious as to what you guys think.

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