I wanted to get a second opinion if the transformer on my ac is dead. I've been all over the net looking up info, and I think I got it figured out. Here's an overview of what was wrong and what I've done:

Two units, one upstairs one down. Downstairs stopped working. No display on digital thermostat.

No 24v at thermostat. Swapped thermostat with the one from upstairs, still didn't work.

Can freely move fan on blower motor on inside unit (in attic) and outside unit.

Can physically push in "thingy" on contactor and unit comes on.

On inside unit, checked for 220 on one side of transformer... good. Checked for 24 on other side, no good. Compared this to the working unit for upstairs and it had 24v.

The next step I was thinking about doing was swapping the transformers but I'm going to wait till early morning when the attic has cooled off.

Is there anything else I should check or did I do my homework correctly? ;)

If 220 is going in 24 should be going out of the transformer.

The next step I was thinking about doing was swapping the transformers but I'm going to wait till early morning when the attic has cooled off.

Don`t bother,, unless you`ve a broken wire,just get a new transformer and replace the bad one

If you have a circuit board inside the inside unit you will a an auto type blade fuse. Check it to see if it has blown

I've looked all over and haven't seen a fuse anywhere

Update: I changed out the transformer, and while I am now getting 24v at the thermostat (multimeter actually says 28v, not sure if that's normal though), the thermostat makes a continuous clicking sound. As does the outside unit. And the unit never actually comes on. Any clue what's going on now?

24 or 28 it is all the same. You have low voltage now but are still having problems. You need to locate the low voltage signal circuit and repair it. The only real question you need to contend with is can you find that problem.

Did you change any wiring during your adventure?
Did you get a transformer large enough (supplying enough VA) to be able to activate your system? 40 VA minimum.
Have you checked the other components in the 24 volt circuit that could be causing the problem and probably burnt out the old transformer to start with?
Is it possible your system was hit by a voltage spike from the utility company? Have you experienced these in the past? Temporary high low voltage. (lights brite or dim) Brown out?
Check for loose or not well made connections in the complete 24 volt system.
The wiring and other problems will need to be solved by yourself with your meter or when all else fails call a qualified contractor and see what they can do.

I'm suspecting an intermittant/loose connection with the 24 VAC. I would jump R to Y temporarily externally on the stat or at the furnace.

Well I had low voltage. Whatever started clicking with the new transformer in seems to have killed that one too. Probably time for a pro.

Sounds like a short. This is probably what caused the other transformer to fail.