The good ole frag-nade is probably the most under-utilized weapon in multi-player.
When you are approaching an up-ramp, and you know your enemy is just waiting at the top of it, a good way to get him to move and perhaps even kill him, is to bounce a 'nade UP the ramp.
You can do this by aiming at the last few feet of the up-ramp as you approach it. The nade will simply skip up over the edge and almost immediately explode in the face of any waiting enemy.
This tactic is especially useful on Colossus and Ascension, when teams tend to try to take and hold the high ground. The nade bounce is particularly effective on the central 'bowl' of Ascension, where the enemy often clusters in Crazy King. Simply stay back and hidden behind the 'lip' of the bowl and lob a couple of nades at the upper edge of the lip of the bowl. They will bounce up and over, often exploding at the head-level of any inside the bowl. A couple of nades followed quickly by a dual-smg/pr-smg assault will often result in triple-kills or kill-taculars if your enemy is bunched tightly in the bowl.