Pentax's P-TTL system includes the ability to control flashes remotely, not just as dumb slaves but with coordinated exposure information. There are two required components: a master or control flash and at least one off-camera slave flash.
The built-in flash on every K-series Pentax dSLR (except the older K100D) can act as a master/control flash — even on the low-end K-r/K-x.1 (Unfortunately, it appears that the K-01 mirrorless camera isn't included.) This means that when I say in this section that a flash can be used as a wireless P-TTL slave, it means you only need that plus your built-in flash else to get this incredibly useful functionality.
Pentax's AF200FG is, unfortunately, not able to act as part of a wireless P-TTL setup. In combination with its lack of a tilt head, this makes it largely useless. If Pentax were to add this feature to a future version, the flash would move from being merely basic to being an entry-point to the whole system, and would remain useful as secondary lights for intermediate users.
Both the Pentax AF360FGZ and AF540FGZ can act as both controller and slave units.
The Metz 44 AF-1, 48 AF-1, and 50 AF-1 can act as a slave, but not as a controller. The 58 AF-1 and AF-2 can do both, but due to a bug in the current firmware, neither model can do HSS in master/controller mode. (They work fine as HSS slaves, if a Pentax flash is the controller.) There are currently no known plans for an updated firmware to fix this issue.
The Metz 48 AF-1 and higher can be set to one of Pentax's four channels. The 44 AF-1 has a notable drawback: it responds on all channels, which could be a problem if you're in a shared space with other Pentax photographers.
The 54MZ-4i can't participate in Pentax's system but with a separate module from Metz can have similar functionality in combination with other Metz flashes in non-P-TTL auto mode — either other 54MZ-4i flashes or more powerful models, or with the 28 CS-2 slave unit (which seems a bit pricey in the US but is interestingly reasonable in Europe).
Sigma's Super versions can act as both slave or controller; the ST versions can't do either. Be aware that in low light when using a built-in flash as a controller, a EF-530 DG Super slave may be triggered erroneously by the autofocus assist strobe used on camera bodies without a dedicated focus-assist light. Pentax and Metz flashes don't have this problem, and it's a non-issue on newer camera bodies which have a focus-assist lamp. (Not sure yet about the updated Sigma model.)
Tumax's DPT5 series flash can be used as a wireless P-TTL slave, but not as a controller.
The Sigma EF-530 DG Super and EF-610 DG Super, the Promaster 7500EDF, and the Tumax DPT3 series flash can be used as dumb slaves (simply flashing in response to another flash), but this requires another non-P-TTL flash on-camera, because otherwise they will be triggered before the exposure by the P-TTL preflash. Promaster also makes a slave module for the 5000-series flashes. This has the same limitation.
With a firmware from February 16, 2011 (or newer), the Metz models from the 48 AF-1 on up can act as what Metz calls "servo mode" slaves, where the initial preflash is ignored and the flash fires at a preset manual level when the main on-camera flash fires. This mode is enabled by putting the flash into SL + M mode — instructions for doing that with the 48 AF-1 or 50 AF-1 can be found here. Normally, one would use this with the on-camera flash not set for wireless.
The Tumax DPT5 series flash also has a preflash-aware manual slave mode, where you can tell it to skip from zero to nine flashes before firing.