At work I occasionally need to work with devices that have serial interfaces, like Cisco Access points, and wanted to do so with my MacBook. One particular USB-to-Serial adapter that I found works OK is a Sabrent SBT-USC1K, which uses the same MacOSX Prolific driver I had installed a while back for use with a GPS. I've used this on both MacOSX Tiger (10.4) and Leopard (10.5).
There are probably lots of USB-Serial adapters that work with a Mac, and even use that same driver. I just thought I'd jot down one in particular, so someone doesn't have to make a wild guess and order something hoping it happens to work with OSX.