Alchemy Immortalis. It's free along with the ornaments. All at the bottom of the tree in presents. Make sure you look around, they have a gorgeous cottage you can get on the Marketplace for half price.
The Garden of Dreams trees are lovely. This is my favorite. It comes in two versions, one full prim, and one that uses temp rezz with 2 prims counting against your prim allotment. I love the textures on this one alot.
Botanical is a great standby. There are different sizes available with and without snow.
Sways tree is festive and has animations and poses built in! Great for you flickr addicts.
Clutter has a few different trees and some great Christmas furniture as well. There was a sale going on, I'm not quite sure when it ends so you might want to hurry over there. Happy shopping!