Originally Posted by tFUnK
cassette, shifters, chain, at least a rear derailleur, and maybe some chainrings.
I'm not sure about the new 6700 ultegra, where they've apparently ruined everything. but you can run the 6600's requisite shifters, chain, and cassette with the tiagra RD with no issues. Also, you'll need to make sure that you have the very-narrow 10speed chainrings. Depending on the vintage of your tiagra crank, the inner ring may already be 10speed-compat. You can save some money by running 105 cassettes; they look the same, perform very well, and no one will ever either know or care. I'd use someone else's 10speed chain, as well. I feel that shimano's current chains are yucky, although many will disagree. Oh, fwiw, your current fd will do nicely.