The only other credible alternative that I've heard of is the Promaster 5750 DX with the Pentax Digital module. A number of users from different forums are happy with them, but AFAIK they are more expensive than the 500 ST. For full P-TTL compatability, Pentax, Sigma, and Promaster seem to be the only players.
I have heard of one guy who bought a Digital Concepts that was supposed to be P-TTL , but the first one died after a few flashes, and the replacement actually blew up! . . . needless to say, I crossed it off the list of possibilities.
Auto thyristor flashes like the $80 Sunpak 383 are a very good option. They can be used with any camera that you can set aperture, shutter speed, and ISO. They have a sensor in the flash which cuts off the flash when it senses that enough light has reached the camera. All you have to do is set the ISO speed and a shutter speed 1/180 or slower, then match your ISO speed on a slider and choose a power range on the flash, then set the aperture on the camera according to what the flash tells you. The flash will then give you at least a close exposure over the distance range indicated on the slider on the flash ( it's a lot easier than it sounds). Film guys relied on these for years with great success, and it's even easier with digital where you can do an instant review and make whatever adjuxtments might be needed. With just a little experience, you'll just know what settings are needed. I use a number of flashes in this mode with my K10 and am very happy with the results. P-TTL is easier, but only a little.