Flare

Overview
This new equipment type is similar in appearance to the Radar Jammer and Power Drainer and is confirmed to make its debut in Halo 3. It functions in a similar manner to a flashbang grenade, blinding foes and allies alike.

There was some speculation among Halo fans that the fourth grenade type (found in the E3 2007 Trailer) was the flare, but seeing how it was announced in the September 2007 Issue of EGM to be equipment, the "Flare Grenades" theory can be dismissed because it's being confused with the "Flame Grenade"

(In refrence to the Flare Equipment by Griesmer): The Flare Merely Blinds rather than hurts, anyone friend or foe alike who sees it-and that's OK. "If you have a short range weapon, the flare will blind foes long enough for you to get close" says Griesemer. His favorite strategy? "See a bunch of people guarding the flag? Throw the flare down." He says "By the time the Flare's gone, their flag is too." Just take care to divert your own eyes-the Flare will blind the player who throws it, too. - ((Quote from EGM September 2007 Issue))