Helvetica is a classic...and a great sans-serif font for clean, modern design. It's probably installed on your computer, but visit Linotype for the full font family.
Stencil fonts are great for creating a stamped look or for a hip-hop style. Download this free one from Dinc Type.
Rosewood is a slick western-style font, great for reproducing the look of old-time posters from the south.