What free, open source, cross-platform alternatives to Skype are there that also don't contain banner ads (although a reminder for donations at shutdown would be acceptable)? Connecting to real land-lines is not needed (although if this feature is available but isn't free, that's also okay).
I like the way Skype implements communications with audio (optional), video (optional), and typing (always available) so that people with any combination of these three mediums can communicate effectively. The lack of banner advertising is very nice too because I don't have to worry about my children seeing inappropriate imagery in banner advertisements when they use it.
The major platforms to be supported should include:
Smart phone support (e.g., Google Android, Apple iPhone {regardless of whether the iPhone has to be unlocked}, BlackBerry, etc.) would be a bonus too, but is not a requirement.
If you're concerned about privacy, I'm not so sure about the benefits of turning to Google over Skype/Microsoft. Probably a better alternative would be using a SIP [1] client.
SIP is a distributed and fully open VoIP and video and presence/chat [2] spec "distinguished by its proponents for having roots in the IP community rather than the telecommunications industry. SIP has been standardized and governed primarily by the IETF[.]" ( Wikipedia [3])
It's roughly comparable to how Jabber/XMPP works and independent of platform, as all you need is a client following these specs.
Google have announced plans to support SIP in the future, so GoogleTalk users will be connected eventually, should user base be a concern.
Talking from SIP account to SIP account is free while routing to landlines is offered by gateway providers such as Sipgate.com [4] at the usual low prices. They offer a plan with "phone number for life, [...] no monthly charges, set up fees or monthly minimum", and incoming calls are free as well.
I've even set mine up to send SMS texts via the command line, if that's your type of thing.
Basically, all SIP clients can talk to each other. You can use soft phones written for your platform or even go for an integrated hardware solution so your PC doesn't need to be turned on and you can use a normal handset phone.
The soft phone I personally use (mainly for its command-line version/scriptability) is Linphone [5] (POSIX, Linux, BSD, Windows, Mac OS X, Android, iPhoneOS):
Linphone is an audio and video Internet phone with GTK+ and console interfaces. It uses the SIP protocol, and is compatible with most SIP clients and gateways. It can use various audio and video codecs such as Speex, GSM, G711, ilbc, amr, Theora, H263-1998, MPEG4, H264, and snow.
I've heard good things about Ekiga [6] and Empathy [7], too, but can't personally vouch.
See also the section on Free and/or open source software on Wikipedia's List of SIP software [8], their list of SIP providers [9] (mainly for external links), and the article on SIP in general [10].
[1] http://en.wikipedia.org/wiki/Session_Initiation_ProtocolI assume you're switching due to today's business deal between Skype and Microsoft. You aren't alone!
An easy one to acquire now is Google Talk. [1] It's quite similar to Skype, with full video calling and chat. Living in the USA, Canada, and some other countries, you can even make calls for free; there is a browser plugin directly from Gmail that will do that. Almost all the features can be used from the site, with a small plugin.
Requires Windows XP +
or Mac OS X 10.4 +
or Linux
Google is definitely leading something interesting here, just from the browser.
[1] http://www.google.com/talk/A major alternative to proprietary Skype, could be a peer to peer network. We have p2p for files exchange, for torrents, for money (bitcoin) and a quick search in google has returned a few attempts in this direction. One of them is OpenVoIP [1]
An uncentralized p2p network for VoIP, texting and video would bring many benefits, but still seems a little bit far.
[1] http://www1.cs.columbia.edu/~salman/peer