Gervase Markham has a nice rant on how complicated SIP is.
Jingle on Jabber is a much nicer solution if you're not a telecommunications person yourself. It's an open standard that's much simpler than SIP which makes it easier to implement, and there are open source libraries available that provide support, and it is Jabberish which is sensible and makes all the confederation work nicely. It addresses some of the technical issues that make people seem to like Skype (getting through firewalls etc) without having some of its headaches (proxying other people's phone calls through your computer, a totally mad idea).
The main issue is that the only current final-release program available with support is Google Talk; it's not open source and its only available on Windows.
I'm currently recommending Google Talk to Windows-using friends in the hope that the best solution will win.
There are also a few emerging services for doing Jingle-to-Phone calling: gtalk2voip seems to work well, I've also seen jabphone. gtalk2voip apparently now also support SIP interoperability (which is only described as currently free of charge).
And it seems like Asterisk Jingle support is on the way too...
In terms of open source support, Patches / Branches are available for Psi, Kopete and Gaim (although that one's a bit more tricky to get working). See my blog on building Psi and Kopete on Fedora Core 4. Neither were too complex, and that was a few months ago.
Unfortunately all of these patches/branches are languishing in we'll-finish-that-at-an-undetermined-date mode, as the projects are busy doing other things and so on. Yet they all seemed to work reasonably well, a lot of the remaining work is cleanup and merging to the main branch etc. (The one most likely to emerge in the official version is Kopete as its in 0.12, which is in Beta. But I'm not sure whether it'll be included in official builds on various distros...)
There are a number of proposals out there to do more work as part of Summer of Code, and I think it would be great if people signed up for these:
Jingle Audio Jingle Video Kopete Jingle Support