How do I eliminate stagnant water caused by central A/C draining outside?
You'll want to create a small leach pit, which will allow the water to drain into the ground.  

Start by digging a hole about 12-18" deep (depending on your soil), Next fill the bottom of the hole (about half the hole) with crushed stone (loosely compacted).  Fill the rest of the hole to about 1" from the top with sand, then finish with a nice decorative stone. If the soil is typically very moist (i.e. you get a lot of rain), you can skip the sand and fill the hole all the way up with crushed stone, and then top with decorative stone.  Another option would be to top the hole off with something like this



So you'll follow the same procedure as above, but rather than filling the second half of the hole with sand/stone you'll put in a drain pipe like this.

The whole idea here is to have the water leach into the ground, without forming a puddle where mosquitoes and other things can breed.

If you do this and still find that the water is pooling, you'll have to take more drastic steps.  The best option from there would be to install a French Drain, and connect the drain line from the AC unit to it.  There are already some questions that deal with french drains, so if it comes to that you should have no trouble finding out how to do it (how do i build a french drain).

You could also buy a small leach pit, but this might be overkill for your situation.  



You'll bury this in the ground surrounded by 3/4" - 1 1/2" crushed stone, then connect the AC unit drain to the lower inlet pipe (the upper pipe is a vent). Before you drop it in the hole, remember to remove the small hole knockouts (to allow leaching). This will require a larger hole, and again may be over kill for your situation.