This question has come up frequently. Here are some answers I have posted before. Most of these are available through Great Models Webstore at http://www.greatmodels.com (no affiliation, just have had great service through them)
Trumpeter CH-47D crew. Set of crewmen and a couple of Soldiers for their CH-47D. They look pretty interesting. The aircrew are wearing the current HGU-56 helmet as worn by the USAF and US Army crewmen. Looks like it also comes with some extras. The Soldiers look like they could be used in any OIF scene as well. Here's the description from Trumpeter:
"Set makes 5 figures and includes equipment, display base and a cockpit for use in making a diorama on the base provided."
And a link to sprue shots and a mini-review.
Vietnam CH-47 Crew. Another announced crew set from Trumpeter.
It includes 2 seated pilots, a standing Crew Chief, and a standing ground crewman. It is scheduled for release in 1st Quarter '08. It looks promising for a nice Vietnam Chinook dio. They look usable as any helo pilot/crew in Vietnam as well.
DML has a pretty nice Vietnam era crew. It comes in and out of production as DML sees a need for them (or something like that). They can be found from time to time on eBay as well, but can go for a high price there. They are now included in the Dragon (ex-Panda) UH-1D Huey kit as well.
Nemrod has a modern UH-60 crew and a couple of guys that look like a crew chief and a AF PJ:
CMK
UH-60 crew. A set of three crewmen and a few GIBs (Guys in Back) listed for UH-60s, but the pilots and crew chiefs have Vietnam era "Chicken Plate" armored vests. I have seen them advertised as a UH-1 crew as well. The soldiers look from about ODS era (late '80s to early '90s):
Same CMK set as a Vietnam era UH-1D/H crew, which they are more suited for due to their "Chicken Plate" body armor.
GIBs
Vietnam UH Door Gunners. Announced new release. I haven't seen them on the market yet. They look promising, sort of. The M60 gunner would be fitting for a UH-1B/C or an OH-6A as a free M60 door gunner. The other figure is a grunt with an M16A1 wearing an M1 helmet, not a door gunner at all.
Seated Vietnam Pilots. Announced new relese. Again, I haven't seen them on the market yet. They too look promising.
US Helicopter Pilot's Vietnam, Standing
While these aren't pilots they'll go well with Vietnam Hueys:
Seated Cav troopers by CMK
Legend US Cavalry Heliborne Set by Legend. Some have found these figures and their gear to be a little undersized.
Verlinden made/makes a standing US Cobra pilot as well. Pretty good figure w/ flight suit and survival vest.
Coree (sometimes under the Dynasty or Legend names as well) has a set of 2 resin figures called UH-60 Helicopter Crew: 1 helmeted male w/ M4 carbine; 1 female wearing cavalry hat. It is kinda hokey, female is in image of Barbie, she doesn't look too realistic. Male is wearing nomex ABDUs (Aviator BDUs) though.
Kirin from China, which is the resin side of Dragon, also had a couple sets, but both are long OOP. One was a set of Cobra pilots from ODS. The second was a set of crew chiefs loading ordinance.
ADV/Azimut has a set of Helo pilots listed as US Helicopter Crew for Huey, Cobra, Apache. They come with both SPH-4 and the ITAS Apache helmet. The set includes one seated adn one standing pilot. They look pretty nice.
Also, some kits come with good crews.
Academy/MRC
UH-1C kits, you get a pretty good 4-man crew from Vietnam era.
OH-58D kits come with a good modern 2-man crew, probably the best modern ones out there. They also come with molded on seat belts as well. Very nice.
Bell-47s come with good Korean war era pilot as well.
(Notice the Academy/MRC trend? Hmm?)
DML/Dragon
OH-6A comes with two guys form the DML Vietnam helo crew set too. The pilot with "WAR" on his helmet (Hugh Mills, actual pilot of Miss Claude IV) and the seated gunner.
LAPD MD-500 comes with a pilot and sniper in the back as well.
Sentinel/SAR
UH-1B/C models also come with a descent 4-man Vietnam era crew as well. Not as nice as the DML or Academy ones, but still very usable.
That about covers it. Not much out there, definitely an area that needs more attention.