I haven't found a free Vst that makes a brass sound that doesn't sound fake. I suggest using a sampler and searching for actually horn samples. There are a few free vst samplers out there if you look.
Ken's Super Duper Band 'n Stuff - Berkeley Social Scene - Tiny Robots - Seamus Collective - Semolina Pilchards - Cutie Pies - Explino! - Bravo Bros. - 2 from 14 - and more!
i would just like to remind everyone that Ken eats kittens - blue lang