There's different versions of this sort of thing marketed. Some just try to cash in on the idea/name. Others are designed for actual use as an impact or stabbing instrument. The more expensive ones use relatively exotic construction matierials in order to avoid metal detectors, corrosion, and what not. Yes, sometimes the real black ops types do use such, however... A fine point Bic - the ones with the clear plastic casing - has long been a favorite for prison shankings as is the humble number 2 pencil - good for more than just scantron tests.
The exotics aren't commonly found and are sometimes bought by Seals (they seem more suspectible to such antics) partially out of an ironic sense of humor and partially for actual use. Some CIA people doubtless buy them if they are fans of Jack Bauer/Burn Notice/James Bond/What not.
I used to carry a Mont Blanc Boheme fountain pen. It was nice for writing, but the nib would bend if you stabbed someone with it. It leaked after that.
Humble golf pencils, which is what they let you have in the mental ward, hurt too if you're stabbed with one in the cheek or eye.
The other exotic pen of note is probably the Fisher space pen (no relation). These are somewhat popular in the military. Not for stabbing mind you. But the nitrogen filled cartridges allow them to write at all angles and under nasty conditions. Several other companies make such, but the best ones are probably made in Germany. Contrary to myth, while they were designed for NASA so that astronauts could write in space (they work fine in Zero G or without an atmostphere), the U.S. tax payers did not foot the bill. Rather Mr. Fisher saw the need and spent his own money to design them and bring them to market as a public service, and NASA simply adopted something that private industry offered. (Some management books incorrectly claim that NASA spent millions to develop these pens whereas cosmonauts just used pencils...).
Staedler Lumicolor Permanent markers - again made in Germany - are also commonly found in the military, amongst PMCs, etc. They're used for nothing more exotic then marking maps and such.
China markers are also sometimes seen, and sharpies are commonly carried by Corpsmen/Medics. Triage and other things dontcha know.
Tactical underwear is in fact sold, though some people still prefer the expensive Hanro of Switzerland Boxers (something like 90 bucks a pair). Mainly "tactical" undies are the same as other exercise products with a few modifications. They're designed to either wick moisture away or else to provide support (ala an athletic supporter). That's in the way of bottoms. Some people still prefer to go "commando" and do without since you lose the support functions, but it is easy to relieve yourself in your pants (just rolls down your leg) and reduces the incidence of crotch rot (at least many feel this is true).
Tops are more common. The latest uniforms from Crye and such have a mesh sort of top around the torso for use with armor. Until that becomes more common, and even used in conjunction with, various T shirt designs to remove moisture/sweat are commonly seen for use with armor. If you've ever put on a set of rifle plates, you'll understand why.
Tactical socks exist too. Mostly they help keep down your foot stink and delay the onset of foot rot. Generally using Gold Bond helps too.
