Thermite grenades have to be abot the coolest "weapon" I've ever had the pleasure to use, albeit in an RPG rather than RL.
Not a lot of blast of fragmentation, but damn do they mess up enemy equipment!
Thermite, as we learnt on Mythbusters over the weekend (same episode Targan saw) is simply powdered aluminium and iron (I think, missed the second one) mixed together VERY carefully - even the heat generated by mixing can be enough to ignite it.
The Hindenberg airship we all know and love ("Oh, the humanity") was coated in paint incorporating both these elements - a big contributor to the catastrophy.