PHP Obfuscator/Obscurer
"It is a free and open PHP program that makes PHP source code almost impossible to read and edit for normal humans."
Take a look at the site:
http://pobs.mywalhalla.net/
It basically takes your code and reformats it, renaming functions and variables to obscure names. The site lists advantages and disadvantages of the product.
It is nice because it doesn't need a decoder to be installed on the server, however it may take a little re-working of your code to get it functional.
The big plus: It's Free.
If you are looking to protect your code and are willing to put in a little effort before shelling out a couple grand, it's definitely worth checking out.
-Mark Fisk ::: CUBEWORKS