Four reasons:
1. Security-- Using whole disk encryption, the machine is well-secured when completely powered off. When on, the key is in RAM and the disk is accessible. This also goes for services that are running.
2. Energy savings-- Why keep a machine using energy, even a few watts, if that adds up to something over the life of the machine?
3. SSD-- My computer boots and halts in about fifteen and five seconds, respectively, only slightly longer than the resume from hibernate.
4. Freshness-- Though rarely an issue, there's nothing like starting with a clean slate each day. No stray processes, memory leaks (FF v4) or conflicts.

Features here: http://www.jitsi.org/index.php/Main/Features

Yes, finally ZRTP (Phil Zimmermann's Zfone prototcol) and OTR. I'd like to see H.323 next, please! It also works over XMPP and Google's gChat infrastructure.

I have to concur with Jitsi being an excellent FOSS (FLOSS) replacement for Skype. The developer has gone multi-platform with Java and some native elements for video and audio. It needs some simplification of the UI, but that's not the hard part. The story of it's development was recently included in a /. story on OSS projects book earlier this week:

http://developers.slashdot.org/story/11/05/23/2227232/The-Architecture-of-Open-Source-Applications
http://www.aosabook.org/en/jitsi.html

http://en.wikipedia.org/wiki/ZRTP

First, don't bet your life on this technology or OpenSSH or other tech.

Second, rather than run TOR on an everyday personal or work computer (Windows or Mac or Linux) with sensitive data and identifiable traits, I'd recommend booting a LiveCD: TAILS (v0.7.1 is the latest) and Liberté Linux:

http://tails.boum.org/
http://dee.su/liberte

or get Knoppix and harden it:
http://knoppix.com/

Change your MAC and connect at a coffee shop (if paranoid-- on the other side of town, and wear sunglasses in case of surveillance), not from home. Or connect to someone else's open WiFi, or get the key with Backtrack. Less secure is running a LiveCD in a VM (virtualbox or vmware). Another less secure option is running a hardened Linux, or at least running the Bastille script.

What am I missing? The main trouble with the LiveCD/DVDs is the NIC driver/module, but Knoppix is good for that.

integral-fellow