One of the coolest features of the just announced Keynote '09 is the corresponding $0.99 iPhone / iPod touch controller application (iTunes link) which allows you to advance slides, see your notes, and even see your next slide on your mobile device's screen so long as it's connected to the same wifi network. Once I had a look at it, however, I was surprised to discover that there were already several similar applications available in the iTunes store which work with Keynote '08 and even have quite a few more functions.
I haven't had a chance to test them all, but there are at least four applications out there including (all iTunes links): Pointer Remote (which also works with PowerPoint) ($0.99), jfControl ($3.99), AirMote ($4.99), and Stage Hand ($7.99). Three of the four require installing a small helper app on your Mac which then interfaces with the iPhone app (AirMote is the exception since it uses the Mac's built-in VNC protocols). Of these, Stage Hand seems to have the most features, including some nifty ones like advancing directly to a particular slide, on-device timers, a highlighter, and a blackout/whiteout function. (Stage Hand presently has limited Keynote '09 compatibility, but they say they're working on it.)
Let us know in the comments what your favorite iPhone / iPod touch based Keynote remote control is.