Any advice on unthreading old Brass pipe?
Try it
Before you assume it's not going to turn, give it a try. You might find that even though the pipe looks like it's in rough shape, it unthreads without too much trouble.

Put a pipe wrench on top of the pipe with the jaws open down, and the handle to the left.
Tighten the jaws down on the pipe.
Try to rotate the pipe in an anticlockwise direction.


Give it a tap
If the pipe won't turn, tapping might break it loose. However, you'll want to be gentle, so you don't break anything. You're not looking to apply extra rotational force, you're looking to knock crud loose from the threads.

Apply gentle anticlockwise pressure to the wrench on the pipe.
Gently tap the handle of the wrench and/or pipe with a hammer.

Two wrenches
If the pipe still doesn't budge, try putting a second pipe wrench in the opposite direction on the outlet of the tee. Then push the handles of the wrenches together.

Put a second pipe wrench on the bottom of the tee outlet, with the jaws open up and the handle about 45° below the first wrench.
Holding the second wrench stationary, push the first wrench handle towards the second.


The second wrench reduces the amount to torque transferred to the fitting, and gives you a bit of extra leverage.
Gunk buster and lube
If the pipe is still being stubborn. Soak the joint in penetrating oil, and allow it to soak in for a bit. Then repeat the steps above.
Heat it up
If the pipe is still not moving after repeating the steps above a few times, It's time for more drastic measures.

Wipe down the joint to remove any excess penetrating oil.
Heat the joint with a torch.
Using a pipe wrench as described above, try to rotate the pipe anticlockwise.

Cut it out
At this point you'll be at your wits end, so it's time to get destructive.

Use a pipe cutter to cut the pipe off close to the tee.
Using a mini hacksaw, carefully make a few cuts into the interior of the pipe.
Using a wrench, try to break the pipe nub up in to pieces.


Be careful when making the cuts, as you don't want to cut into the threads of the fitting.  Usually you can break or deform the pipe nub enough, so that it can be removed from the fitting.