Letters of Note is a pretty cool site that posts personal correspondence from celebrities or historical figures, usually as submitted by the letters' recipients. Some are short missives between colleagues, some are fan mail (there's a particularly touching one from Wil Wheaton to an 8 year-old fan club applicant, 20 years late), others are fairly major pieces of secret history that were seemingly lost over time (like the memo from JFK to the head of the then-fledgling NASA, asking how the U.S. could "win" at the space race).
Below is one I like a lot, an answering letter from Kurt Vonnegut to another author just getting his start. Mark Lindquist had sent Vonnegut a copy of his first novel looking for feedback or advice from a role model. Very cool signature and self-portrait. A more internet-friendly transcript is linked below the picture.