I put together the following list a ways back – don’t know if they are all current, or how many are now free, but you should find something useful here:
About:
Software:
- Safeyar (Best)
- PHP’s APC (PECL page)
- RoadSend
- Turck-mmcache
- eAccelerator
- PHP-Accelerator
- SourceGuardian
- NuSphere Nu-coder
- Gridinsoft
- IonCube SA-Encoder.php
Another thread on SO that adds a few more (check it out):
Can I encrypt PHP source or compile it so others can’t see it? and how?