"GnuPG is a complete and free replacement for PGP. Because it does not use the patented IDEA algorithm, it can be used without any restrictions. GnuPG is a RFC2440 (OpenPGP) compliant application." -- http://www.gnupg.org

The GNU Privacy Guard is the open source alternative to PGP, the Pretty Good Privacy encryption tool. It's designed to support a lot of encryption algorithms, including; ElGamal (signature and encryption), DSA, RSA, AES, 3DES, Blowfish, Twofish, CAST5, MD5, SHA-1, RIPE-MD-160 and TIGER.

How the GPG works:
There is a private key generated, and there is a public key generated. With the public key, you encrypt the message to be transferred to the owner of the private key. This means that only the owner of the private key can read what you are sending him/her. The private key must be kept secret, while the public key should be spread to those you wish to have encrypted conversations with. The private key is the only key that can decrypt the contents of the encrypted message, so care for it wisely.

This is also more or less the concept of PGP.

Supported systems/platforms are:

GNU/Linux with x86, alpha, mips, sparc64, m68k or powerpc CPUs.
FreeBSD with x86 CPU.
OpenBSD (x86 CPU?).
NetBSD (x86 CPU?).
Windows 95/98/NT/2000/ME with x86 CPU.

AIX v4.3,_BSDI v4.0.1 with i386,
HPUX v9.x, v10.x and v11.0 with HPPA CPU,
IRIX v6.3 with MIPS R10000 CPU,
MP-RAS v3.02,
OSF1 v4.0 with Alpha CPU,
OS/2 v2.
SCO UnixWare/7.1.0.
SunOS, Solaris on Sparc and x86,
USL Unixware v1.1.2,