If you're looking to control Keynote presentations and your Mac doesn't have a built-in IR port, you could buy an external receiver, but I suggest you use either:
The Keynote Remote app for iPhone. This works over wi-fi and requires only a $0.99 purchase and no additional hardware to buy or set up.
http://a1.mzstatic.com/us/r1000/117/Purple/91/59/26/mzl.tpdgngil.320x480-75.jpg
or
A specialized slide advancer, like this Logitech one. This is designed to be comfortable to hold (the Apple Remote wasn't) and doesn't require line-of-sight to the receiver (which is a small USB dongle) to work. Some of the better ones have a slide counter and laser pointers built in.
http://www.logitech.com/assets/16078/16078.png
