I use Thunderbird for email on my laptop running Ubuntu. It works great, but the default skin isn't to my liking. I've tried skinning it with mixed results, but today I've found The Skin I've Been Looking For (but not in a creepy, Buffalo Bill kind of way).
Deviant Art user ~Rio-2007 has posted a very convincing Mail for Leopard skin. After download, simply apply the skin via Thunderbird's Add-Ons option (Thunderbird 2.0 is required).
Try it out. Because even our non-Macs must resemble Macs, not that we're obsessed.