Web site: SCA Adapter 3702
Key Features: AF assist beam, enables P-TTL.
Reasons to Buy: Required for the most powerful P-TTL compatible flashes, which could be shared across systems.
Notable Drawbacks: Doesn't integrate quite as well as a system-dedicated flash.
This isn't a flash, hopefully obviously. It's the Pentax-compatibility module used with the 54 MZ-4i or the more powerful "hammerhead" flashes. If you get one of those those, you'll need this. This may be an especially appealing option for people with multiple camera systems: you could share the same flash between Pentax and Canon, Nikon, Sony, Olympus, or even Leica.
More about the Metz SCA 3702 can be found on these pages:
Including an Amazon link on this page is the easiest way to display the current prices, without having to keep track of it manually or write a more-complicated tracking script. And as a bonus, there's a picture of the flash. Obviously, the ad is also an affiliate link. As with all advertising on this site, I hope it's not too obnoxious.