How about...?
Turn and Fire RR
Instant
Untap target creature, it gains First Strike until end of turn.
"When I count 3, fire! 1...2...4! Er 3!" Pab, Goblin Liege
Binge XUU
Instant
Play ~this~ only during an opponent's turn.
Target player must tap X lands and add colorless mana to his or her mana pool for each land tapped in this way. If that player does not spend all mana generated in this way by the end of his or her turn, he or she loses X life, but does not take damage from mana burn.
"Make him eat his words." - Reigh, Psychemage
Pulled Punches 2W
Sorcery
You may choose not to declare an attack this turn, if you do so, opponent cannot declare an attack during his or her next turn.
"Whoever said the best offense is a good defense was out of his mind. The best offense is no offense!" - Kaiyer, Pacifist Priestess
Sutures B
Instant
You may pay 1 life instead of paying Sutures mana cost.
Regenerate target creature. Put a -1/-1 counter on that creature.
"What we need now is efficiency, not quality." Koval, Heartshade
One Fell Swoop 1BB
Sorcery
Destroy all creatures in play.
Each player may pay 2 per creature to prevent that creature from being destroyed this way.
"Will Koval never again see the reason he once followed? His wake of destruction saddens the clergy." - Kaiyer, Pacifist Priestess
Leverage GG
Instant
All creatures you control get +1/+0 until end of turn.
All creatures target opponent controls get -1/+0 until end of turn.
"An advantage is an advantage, I'll take whatever I can get" - Fotinde, Elvish Warrior Lieutenant
Most of these aren't too great, but it's late
And I have so many more...
D