Fighter player character attacks with a grenade-like weapons, e.g. Acidic Fire [ddi], can they still
Mark the target(s)?
From PHB1, pg76:
Combat Challenge: ... Every time you attack an enemy, whether the attack hits or misses, you can choose to mark that target. ...
Note that there are no restrictions on this. (Hybrid fighters are slightly different and require the use of a Fighter class power to mark a target.)
If the grenade-like weapon requires an attack roll (and all the ones I've seen do), then yes, the fighter can mark the target(s).