Letters of Note: Correspondence Deserving a Wider Audience is a wonderful collection of letters worth reading. Inspiring, funny, confused, and courageous.
Letters of Note is an attempt to gather and sort fascinating letters, postcards, telegrams, faxes, and memos. Scans/photos where possible. Fakes will be sneered at. Updated as often as possible; usually each weekday.
Reading such wonderful missives might help inspire you to write, or absolve you of guilt because others are doing it so nicely.
I discovered this site when Nora Ephron died and Elizabeth Wurtzel, author of Prozac Nation, submitted one of her letters to the archives:
There are regular additions. Book forthcoming in November 2012.