Most models have a built-in wide-angle diffuser which pops out and flips down over the flash head. Exceptions are the Promaster flashes and the most basic Metz and Tumax units. Most Promaster units include a push-on diffuser in the package, as does the Metz AF 36 AF-5. (The earlier Metz 36 AF-4 did not.)
These diffusers are not designed to soften light (like a mini-softbox) but simply to disperse it for use with wider-angle lenses. This is necessary at real focal lengths (on current Pentax dSLRs) below 16mm for the zoom models and below about 19mm for the fixed reflector models. Although the exact claimed numbers vary, using the diffuser should provide coverage down to approximately 13mm (again, real focal length).
Of particular note, the wide-angle diffuser on the Metz AF-2 covers an unusually wide angle — a field of view given by a 8mm lens on a Pentax APS-C camera.
