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 is some speculation that the fourth grenade type (found in the E3 2007 Trailer) is the flare, but seeing how it wasn't announced as "Flare Grenades," we can assume it is equipment.

Flare: 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.