At mid-low levels, the mage in the party I DM came up with something pretty nasty: the Vampiric Otter Of Doom!
The only exotic component here was the otter, a familiar. The party was engaged in melee, and the otter (for some reason) was the only one that could engage a distant enemy. First round, mage casts
true strike, choosing the otter as the recipient (as you can). Second round,
vampiric touch, again choosing the otter and letting it do the attack.
I thought that was cool, so I allowed it. It certainly put the mage back in good health.
And I came up with this one over on the
How Would A Demon Queen Defend Her Palace? thread:
Pit trap in a corridor. Deep, deep hole. If you look at the ceiling above it, you see some peculiar markings. If you look at the bottom, you see identical peculiar markings. As it turns out, they're linked
teleportation circles. If you fall down the hole, you hit the bottom, take damage, get ported, and the next round find yourself at the top again and fall and take damage. The only problem I see with this is that some DMs might not allow a
teleportation circle on the ceiling, but I say, it's vicious enough to work. It's not actually a combo, but it certainly isn't the accepted use for a spell...