The most popular items here are the 8 foot floor to ceiling cat trees from Drs. Foster and Smith
You can see them here: http://www.drsfostersmith.com/product/prod_Display.cfm?pCatId=524 and here:
Another favorite is a 3 foot tall sisal post from Petsmart.
A four tier cat tree from http://www.createacatcondo.com is also very popular with the cats. Pictured to the right of the tree is another style of floor to ceiling tree from Drs. Foster & Smith.