Matthew Miller has collected every piece of information on Pentax, Metz, Sigma and other P-TTL compatible flashes on his easy to navigate and useful wiki-like website, which also includes the thorough flash related reference. Here is his own description of the project:
I’ve written a guide comparing in detail the current Pentax flash offerings and P-TTL-compatible third-party options. If you’re not sure what to buy, or even what’s available, this will answer all of your questions, including ones you might not have known you have.
I’ve gathered information from many diverse sources, collected it all together, and presented it in several different useful ways. Not only are there easy guides to the top-level features like flash power and tilt/swivel reflectors, there’s a lot of detail on less obvious (but important!) distinctions that most people don’t learn about until after they make a purchase.
I wanted to make sure everyone is able to get the most out of their equipment. Different photographers have different needs, so there’s no one answer to “what’s best for me?”. This guide makes sure you’ll know the answer to that question before you buy.
And, even if you already own a flash, you might learn something, either about features you have but didn’t realize were there, or about features you didn’t know you were missing.