Coolant leaks even without running engine - most likely source?
 

I've got a 99 blazer, which I originally thought had a heater problem because it wouldn't blow hot air. Then I realized it had lost coolant, so I filled it up - fixed the heater problem. But the leak has accelerated and now when I top off the radiator, I can see water dripping from underneath without the motor even running. It appears as though it's coming from the front of the motor. Is this most likely a water pump?

Possibly. The best way to really know is to put it up on some jackstands, get under there, and look. Leaks can be deceptive when it comes to pinpointing their location. It might look like it's coming from one location, but actually be in another and flowing down the block to the point where it actually drips off.
I was at O'Reilly's Auto Parts tonight, and I noticed they had a two ton floor jack with two jack stands for $49.99. You might need to put a wood block on the floor jack to raise it high enough, but that seems like a pretty good price.

My first guess would be intake manifold gaskets. That's a *VERY* common problem on the 96+ 4.3L Vortec V6es. Check the front of the motor right where the intake manifold meets the engine block. That would be a very likely source of coolant loss.

If it is losing coolant due to a leaky intake manifold gasket be sure and check your oil as well. If the level is way up and looks like a chocolate milkshake then it's incredibly likely that your intake manifold gasket is leaking.

If you don't find anything from that inspection then keep looking around for other things: leaking hoses at connections, corroded freeze plugs, etc.

I've checked the oil - looks fine. Would an intake manifold gasket leak like a sieve even if the motor isn't running? I'll check there this afternoon, but I figured it was elevated enough that it wouldn't leak unless the coolant was being forced through. That's why I thought maybe the water pump since it sits lower.

Open the hood and look, you may have to get underneath. Could be a hose, water pump, radiator, intake gasket. Any of those "can" leak with engine off. Remember, things roll down hill. Find the furthest "up" part of the leak and that's where it's coming from.

When you find an area with anti freeze on it dry it off. This will help pinpoint the leak

Man I had to do my intake gasket on my 2002 that has 98k on it... My wallet still is in pain. Good luck dude.