Not much here yet, but I'll toss in what I've got.

Surprise: Knockout

I always liked the idea of being able to knock someone out from behind if they're unaware. It makes for good plot devices, plus it always annoyed me that the only "viable" way to do it in D&D was to beat on them with nonlethal damage until they dropped. So, I implemented the following-

If your opponent is completely unaware of your presence or is rendered helpless, you may make a nonlethal attack on them(such as with a sap, unarmed strike, or a weapon used to deal NL at -4 to hit). If successful, you may forgo dealing damage to force them to make an immediate Fortitude save DC 15 + nonlethal damage that would have been dealt. If they fail the save, they are knocked out for a number of rounds equal to the amount of NL damage they would have suffered. If the save succeeds, they take half damage.

A character (or target) with the Endurance feat is allowed a second Fortitude save. A character (or target) with the Die Hard feat is allowed a third Fortitude save. This is to reflect the extraordinary resilience that these feats imply.

Surprise: Coup de Grace

If your target is completely unaware of you, you may make a grapple attempt during the surprise round and substitute the initial damage for a pin attempt. If the grapple attempt or the pin attempt fail, the target gets an immediate AoO against you and combat begins normally. If not, you are then able to perform a Coup de Grace on the target as a standard action provided he remains pinned until your action. This CdG attempt can be done unarmed or with a light melee weapon (armor spikes do not count).

Note: Under normal circumstances the CdG attempt, being a Standard Action, will have to wait until the first full round of combat unless something such as the Heroic Surge feat is employed. Also, credit to The Shocel, one of my players, for finally giving me the flash of inspiration needed to fit a mechanic to this concept.