If you are writing an application (browser, email, server, etc) that requires the use of either Secure Sockets Layer (SSL versions 2 or 3), or Transport Layer Security (TLS version 1), you can obtain this free open source "commercial-grade" OpenSSL library. The library allows you to add cryptographic functionality to your programs, apparently both server programs as well as SSL supporting clients. You can use it in both commercial and non-commercial programs. This is the successor to the well-known SSLeay library. (Trivia: in case you don't see the connection, OpenSSL = Open plus SSLeay minus eay where "eay" stands for Eric Young, probably his initials.)
OpenSSL
