This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
Nothing! (Compare to ViaCrypt PGP at $98!)
It should be noted, however, that in the United States, some freeware
versions of PGP *MAY* be a violation of a patent held by Public Key
Partners (PKP). The MIT and ViaCrypt versions specifically are not in
violation; if you use anything else, it's your risk. See below
(question 1.6) for more information on the patent situation.
Also, the free versions of PGP are free only for noncommercial use.
If you need to use PGP in a commercial setting (and you live in the
United States or Canada), you should buy a copy of ViaCrypt PGP.
ViaCrypt PGP has other advantages as well, most notably a limited
license to export it to foreign branch offices. See below, under
question 1.10, for information on how to contact ViaCrypt.
If you need to use PGP for commercial use outside the United States or
Canada, you should contact Ascom Systec AG, the patent holders for IDEA.
They have sold individual licenses for using the IDEA encryption in
Ascom Systec AG
++41 64 56 59 83 (Fax ++41 64 56 59 90)