Originally posted by Clicker probably Nikon SB400 or Canon 270 but there are some little LED ones looking for examples.
I could be wrong, but I think both the Nikon and Canon flashes are dedicated to their respective brands' digital flash systems (I TTL and E TTL only, but they might possibly fire at full power only when mounted on another brand cameras).
At present, AFAIK, the Pentax AF-200FG and Metz 24 AF-1 are the smallest P-TTL flashes on the market. Both share about the same features, but the Metz has a tiltable reflector, so can be bounced, but doesn't swivel, and the Pentax cannot be either tilted or swiveled. The Metz is @ $90 USD, and the Pentax is @ $140 USD last time I looked.
Pentax AF-200FG P-TTL Shoe Mount Flash 30465 B&H Photo Video Metz mecablitz 24AF-1 Digital Flash for Pentax MZ 24317PS B&H
The smallest Auto Thyristor flashes (non-dedicated auto exposure flash units where the exposure sensor is in the flash, and the only communication between the camera and flash is to fire) are the Metz 20 C-2 and the smallest of all, the Sunpak PF 20XD. Of these, the Metz is basically an Auto Thyristor version with the same basic body as the 24 AF-1, with the tilting reflector, but no swivel, and the Sunpak is about the size of a pack of Camels (the small unfiltered cigarettes), and cannot be tilted or swiveled. The Sunpack also can act as an Auto Thyristor Optical Digital Slave, which means that you can set it to ignore the preflash in a digital camera's flash system and trigger off the second flash, so you can use it as a wireless external flash, plus it has a sensor in the flash that can cut off the duration automatically to give you proper exposure once the camera's ISO and f stop value are set up correctly.
Auto Thyristor flashes need the user to use predetermined ISO and f stop settings, and can usually be used at predetermined ranges of distance from the subject. It has 3 ranges of power in Auto, and can be manually set at full, 1/4, 1/8, 1/16 and 1/32 power.
Metz mecablitz 20 C-2 Auto Flash MZ 20220 B&H Photo Video Sunpak PF20XD Auto/Manual Digital Slave Flash PF20XD B&H Photo
There might be others, since once I got my Q, I went out and got both of the Metz flashes to play with. I already had two of the Sunpaks that I've used as wireless slaves when shooting macros.
The Q has some quirks in it's external flash functionality. Auto Thyristor or Manual flashes won't fire unless you turn the flash off in the flash menu with Q system lenses (I don't know about the Toy Lenses), and they won't fire at all with adapted lenses at this time. P-TTL flashes seem to work fine with Q system lenses, but will only fire in a limited P-TTL mode in Slow Sync and Red Eye Reduction modes in the flash men when used with adapted lenses. Also, flash sync with adapted lenses (and the FE and "toy" lenses) is limited to 1/13 sec supposedly because of the electronic shutter. This should change (for adapted K mount lenses) with the Pentax branded K to Q adapter, which is supposed to have a shutter built into the adapter. Hopefully, we'll see flash sync up to 1/250 like with the Q Quality Line lenses (01 Prime and 02 Zoom).
I've written to Pentax Imaging Tech Service about this, but got a vague answer saying that the Q was designed to work with P-TTL flashes. . .and they would forward my issues to Pentax Japan. . . I responded that firing a non dedicated flash with any lens is minimal expected functionality for any camera with a hotshoe. . . A enthusiast-featured camera like the Q should, IMO, actually allow wireless TTL with the popup flash as the master or controller, but that might be too much to ask for. Hopefully they'll work on the firmware by the time they release their own K to Q adapter. . .
Sorry for the probably confusing post -- flash is a pretty complicated aspect of photography nowadays. . .
Scott