We use Ableton Live to run our click. If a simple click is all you'll ever need, you may want to look at something more simple like a Dr Beat. But using Live gives us the ability to also run loops (backing tracks) and even runs our light show (another post).
We run Ableton from a macbook and output using the built-in audio port. Nothing fancy.
In order to avoid adding an external interface, we keep our loops/tracks mono. That lets us output the click and count off to the right channel and send that only to our in-ears. Then we send the left channel ("L-eft for L-oop" is easiest to remember) to the FOH console the same way we send any other instrument. * Always test this before a show. We almost always plug in the cables backwards!
We set up a template which we start with for every show. The template includes a 2-bar count off for every song. So we hear a lady say "1 2 3 4 1 2 3 4" and then we start the song. We set up shortcuts to each song so that our keyboard player (or drummer) only has to hit one key on the keyboard to trigger each song.
Each scene (row) in Live is a different song. By changing the title of each scene in the Master column, you tell Live to change its tempo for that song. This lets us cut and paste songs and just change the BPM to get the desired tempo of click track. So essentially, every song without a loop is identical.
Rather than try to explain every detail of this, I'm attaching a couple of screen shots and a blank template I made up, including the count off and click wav files. You should be able to load this up in Ableton 8 and start using it right away. I made sure it was less than 6 songs so that it will still load in Ableton Lite (Lite has a 6-scene limitation). Try it out and let me know if you have any questions.