For some flashes, a wide angle diffuser is a built-in feature. The Promaster flashes don't have that, but the 5750DX and all the 7000-series models include a push-on plastic diffuser in the package. Be aware that this isn't meant to be a light softener or even to provide a bare-bulb effect like a Sto-Fen diffuser — the point is to spread out the flash's coverage for wide angle lenses
A soft carrying case is included with all Pentax and Sigma models, and with the Metz MZ 54-4i and 58 AF-1/AF-2. The Tumax DPT3 and DPT5 flashes usually have this too, but some rebrandings may not.
The Pentax AF540FGZ, Metz 58 AF-1/AF-2, Sigma EF-530 DG and EF-610 DG (both ST and Super), and Promaster 7500EDF models all come with a flat plastic stand to hold the flash on a table or other flat surface for off-camera operation. This is worth mentioning because a stand is incredibly useful for the wireless P-TTL-capable models which don't include it (the Pentax AF360FGZ and Metz 48 AF-1), and getting one separately is more pricey than it should be — see the section on generic accessories.
The Tumax DPT586AFZ includes a stand, but it's not a guarantee that all branded versions of this flash will; make sure to double-check.
The built-in or included diffusers aren't really meant to provide nicer light. They're just there to help cover a greater angle than the widest zoom setting. An optional flash diffuser is generally larger and designed to have a more versatile effect.
Metz makes a couple of flash diffusers designed for their flash units. The Bounce Diffuser 54-23 is a soft, velcro-mounted bounce diffuser that is described as matching all three of the flash units mentioned here and would probably fit on anything. There's also a plastic clip-on diffuser, Mecabounce 58-90 for the 48 AF-1 and 58 AF-1 models and Mecabounce 44-90 for the 54 MZ-4i.
Of course, there are many third-party diffusers like the famous Sto-Fen Omni-Bounce which will fit any of these flashes. However, the 48 AF-1 manual mentions that it automatically detects when the Mecabounce 58-90 is attached and displays a wider focal length automatically — that probably doesn't work with third-party diffusers.
Sigma also makes a bounce reflector designed for their flashes, but which would probably fit on anything of a similar size.
External Power Packs
The Pentax AF540FGZ's recharge time can be improved with the TR Power Pack III, which uses six C-cell batteries.
And the Metz 58 AF-1/AF-2 and Metz 54 MZ-4i can both use Metz's NiMH-based Power Pack P76. They require different power cables, though.
Since the Metz 54 MZ-4i is based on the SCA system of interchangeable modules, it has a whole little universe of add-on possibilities. This would make particular sense if you use multiple camera systems (e.g. also Canon or Nikon) or if you have some of the other SCA-capable Metz flashes.
Promaster's 5000 series is also based on a (different) modular system.
Pentax makes a variety of off-camera flash brackets, hot shoe adapters, and connection cords, which you can see at the Pentax web store. The most interesting one, though, doesn't appear to be marketed in the US — the CL-10 off-camera shoe clip, which is a flash bracket attached via a ball joint to a large clip strong enough to mount the flash anywhere. Oh, and the just-plain-plastic flash table stand is $22 shipped.
Metz makes a wide selection of diffusers, filters, brackets, mounts, and cables, as does Promaster. Sigma's flash stand is the cheapest at twelve dollars, but I think the best bet is Nikon's AS-19 Speedlight Stand, which is designed to either sit on its own or attach to a tripod or light stand — and which is readily available for under $10. I have one and it works very well. The only potential drawback is that Nikon's locking pin goes in a different place than Pentax's, so the pin stays retracted rather than doing its job. The fit seems snug without it, though.