Hisham Marmin's Macro rig. You can find his blog here, and his Facebook page here.
You can also find more macro rigs here.
His macro rigs consists of Pentax K5, SMC Pentax D FA Macro 100mm F2.8 WR3, Pentax AF160C Macro Ring Flash. However, instead of using the ring flash the way it is intended to be, i.e mounted onto the front of the lens, this is what he does. The images here should be self explanatory!
Side view. Yes, it's another great example of adapting the concave diffuser I use for my MP-E65/MT-24EX for a lens with bigger working distance.
You can also see that he uses a wooden ruler, a few rubber bands, and surgical tape to put the whole rig together.
You get O shaped catch light in the subject's eyes if you use the ring flash the conventional way i.e directly on the front of the lens.
However, by holding the ring flash on a ruler and diffuse the light with a concave diffuser, you get very nice, even light like this!
Other sample images:
More macro rigs here.