I'm looking for a free, open source API for encrypting and decrypting PGP files. I'd like to use it in closed source products, as well as open source ones, so it would have to be a license other than the GPL.
Bouncycastle is a library that is very useful for doing encryption work.
Searching for Bouncycastle + PGP turns up this blog post, which looks very useful. It's in Java, but the C# port is nearly identical to the Java code: http://cephas.net/blog/2004/04/01/pgp-encryption-using-bouncy-castle/