Letters of Note is one of my favourite sites. They post some awesome written correspondence between really interesting people who are mostly famous for some or other reason, and they include transcriptions too. Here are some of my favourites:
Johnny Cash to his wife, June:
Hey June,
That's really nice June. You've got a way with words and a way with me as well.
The fire and excitement may be gone now that we don't go out there and sing them anymore, but the ring of fire still burns around you and I, keeping our love hotter than a pepper sprout.
Love John
Steven Spielberg to George Lucas:
Thom Yorke replying to fan mail:
A letter from Albert Einstein to a friend Eugene Simon, making his opinion of astrology known. Pretty funny:
Dear Sir:
I fully agree with you concerning the pseudo-science of astrology. The interesting point is that this kind of superstition is so tenacious that it could persist through so many centuries.
Very truly yours,
Professor Albert Einstein.
A letter to Andy Warhol from his rental agents concerning the parties he had in his studio in New York:
Go have a look at the site, it's really great.