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.