I'm pretty sure these are my block drains, but what is on them? A temp sensor, or a knock sensor? I'm assuming just pull the brass plug to drain my block, on each side? I'm going to winterize later on this month, but I'm trying to get everything all set up so I know exactly what to do.

I'm not positive, but that looks like an oil pressure sender based on the location.

Yeah that should be your knock sensor. To winterize it do just like you said pull the brass fittings off and let the water drian out. While it is out might be a good idea to probe in there w/ a small screwdriver to see if there is any rust/gunk stoping the water from flowing out. No need for a cracked block.

Knock sensors are usually up on or near the heads, but??? My guess is that it's the temp sensor that makes your beeper beep if your too hot...

I agree with FR: that's a temp sensors. It's definitely not a knock sensor.

Thats what I thought, a temp sensor....but why would my 5.7Gxi have three? One on each side of the block, and one on the thermostat housing?

Blockage on one side could overheat one side. Help shut down before too much damage is done. Also, the ECM might be monitoring temp differences which could help determine how much fuel is delivered to help control combustion temperatures.

True. Who knew my boats ECM would be more advanced than my trucks. As far as I know I only have one temp sensor on my truck.

Im speculating. I don't know for sure. Also, I bet the truck has more than one temp sensor. But, what kind of company uses a MAP system instead of a more efficient MAF???? wtf?

Back to boats now.

On my 1997 Mercruiser, it has one of those on the starboard side. And it is most definately a Knock Sensor. Not a temp sensor, sender, oil sensor, or oil sender. You know how I know? Cause I unhooked it, ran the boat without it, and oil pressure guage operated, so did the temp guage, and the motor reached full throttle and ran normally.
It's a knock sensor.

I know there are multiple knock sensor designs...but the ones i've been around fit flush.

Without a shadow of a doubt, it's a knock sensor.

The experts have spoken!

Why would a knock sensor be mounted so low on the block and have a plug in it as if to drain a fluid?

The cylindrical shape of the body looks like it might be holding a spring loaded diaphragm to operate a set of contacts.

Look at parts 64 and 26.

The diagram shows the connection for the knock sensor up by the manifold/head area. The sensor in the photo is down by the oil pan.

the plug is there to drain the block, the knock sensor is mounted there as a matter a convenience it just needs to be attached to the block to work. It really doesn't matter where.