Ubuntu. Or possibly Xubuntu.
The difference is the user interface: Ubuntu uses Gnome by default. Xubuntu uses XFCE, which has a few less features and is a bit faster on older machines.
There's also Kubuntu based on KDE...but at least until KDE 4.0 comes out and might be worth a look.
Gnome is broadly similar to the Mac user interface. I had a chance to compare it with OS10.39 and there were obvious similarities...moreso than KDE for sure.
Ubuntu/Xubuntu version 7.04 "Fiesty Fawn" is really, really good code. Very stable. Auto-updates don't do anything stupid or alarming. Software selection rocks.
In my opinion, Gnome needs the following: P3/750 or so CPU and 384megs RAM. Anything less than that, I consider Xubuntu. Anything at that point or above, I think Gnome is nice enough to stay with.
One minor issue: by default, if you ask for Thunderbird you'll get ver.1.5 series (the latest with all security updates). There's a way to get 2.0.0.6 easily enough. Like so:
Start by opening a terminal window and doing:
sudo gedit /etc/apt/sources.list
That takes you to a text editor. Add the following two lines as two NEW lines:
Save that, exit the editor (it's similar to Windows Notepad).
Back at the terminal, do:
sudo apt-get update
Finally, do:
sudo apt-get install thunderbird thunderbird-gnome-support
Also available at this repository is a set of PGP support tools if you're into encrypting mail:
sudo apt-get install enigmail
You can also install alternate language stuff, let me know if that's an issue.
If you wanted to, you could also go straight to the repository's website:
http://ubuntu.iuculano.it/dists/feisty/thunderbird/...and download and manually install any of the .DEB files listed. The problem with doing it that way versus how I've done it above is that doing it "the proper way", if the manager of that repository ever posts a 2.0.0.7 or whatever version, Ubuntu will auto-update from it. And yes, it's a trustworthy repository
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I'll post more Ubuntu tips here tomorrow
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