ION Cube Encoder - $199
Roadsend Compiler - $399 .. Although it actually compiles the code instead of just encoding it down to bytecode. Slightly expensive, but looks pretty cool.
PriadoBlender - Free compiler.
Zend Encoder - Looks to be the most pricey. Probably only worth it if you're a high-volume dev company or want to have it for internal enterprise use.
I think there are a few more, but I can't find them right now. I'll let you know if I do.
-r