A friend of mine works as a "cornerworker" in motorcycle racing; these are the safety personnel and "directors" of a race. Obviously a lot of their job involves flags, and he described a couple to me. Cornerworkers at this particular track in Maryland where my friend works are trained by MAR/RC (Mid-Atlantic Road Racing Club) for two different sanctioning bodies: WERA (Western-Eastern Racing Association) and CCS (Championship Cup Series). No idea of any applications outside of these organizations, but I would expect from his description that motorcycle and car races in North America and internationally would use a similar set of flags if not the same.
Note that the flags have different meanings depending on how they are waved! This is the only set of flags I can recall where this is the case.
Yellow Flag: Standing (held stationary with free hand) indicates hazardous condition or slow-moving motorcycle on track. Waved indicates hazard of a serious nature on track; no passing allowed.
Debris Flag (ten horizontal stripes, red and yellow): Standing indicates debris on the track. Rocking (waved up and down) indicates slippery condition on track. Folded, with upper fly corner to lower hoist, stripes vertical facing the riders, indicates rain. Pointing (showing flag to rider, then using the pole to point to his motorcycle) indicates a motorcycle with a hazardous condition, such as an oil leak.
Ambulance Flag (white with red couped cross): Standing means an emergency vehicle is on the course. Waved indicates the emergency vehicle is needed at that point.
Blue Flag (blue field, thin yellow stripe from lower hoist to upper fly): Displayed to a rider who's about to be passed.
Red Flag: Always waved. Race is stopped at once due to emergency. The use of this flag is so carefully controlled that cornerworkers are not permitted to say the word "red" on their radios; they must use "crimson" instead. (For example, "Control, this is Station 1; there is a spectator with a crimson shirt leaning over the railing onto the course," or "Control, this is an emergency...the track is blocked. Request that the race be stopped!" If the controller concurs with the latter request, sie will broadcast, "All stations, this is Control: red flag, red flag, red flag!")
Checkered Flag: Waved. Pretty obvious to all. (Anyone know the origin of this?) Not used by cornerworkers, to my knowledge.