Punisher has the following recommendation for solenoids:

"Humphrey's H040-4E1......
Great little valve, have used several with great success.
fittings direct into body
5V/50ohm coil
up to 125psi (I've run them up to 250 with little problem."

Here is a .pdf file with more info on the valve.

Recently, Punisher's favorite Humphrey has become the 401 series, as the H040-4E1 has a low cycle speed (he's managed to push them to just barely 10 cycles/sec, but it required pushing the edge of the allowable pressure and voltage).

Note that Humphrey valves appear to actually be made by Koganei.

John 'SLICEnDICE' Schmitz donated these SolidWorks models of the Humphrey 401 and the Humphrey H040-4E1.

The Skinner (owned by Parker) K4H01 is the valve used in the Tribal, BM2k, Defiant and others.  The appropriate section from the Skinner catalog is here. Not being very happy with the quality of the graphics of the Skinner supplied .pdf, I scanned in the appropiate pages myself here.

Note that Skinner valves appear to actually be made by Kuroda.

Defiance noted that 'Hosheta' valves used in later Intimidators appear to be Skinner valves. Unfortunately, I can find no mention of 'Hosheta' anywhere other than the Intimidator manual.

CRySyS notes that the Rainmaker uses a model 44B-AAA-GDSA-1BA valve from MAC.

Edward Kang found that Sandridge's Force5 cocker uses MAC 44A-AAA-GDSA-1KA solenoid valves.

E-Mag uses a Pull Type Tubular Solenoid, 26 AWG, pictured here. You can get more info at SolenoidCity.