The iPod is most certainly not a PDA, you are correct in that. It's a music player with a couple extra features.
Palm, Handspring, and Sony all make excellent OS X-compatible PDA's, and they all use the practically de-facto Palm OS. All synchronize with OS X via iSync or with Palm Desktop, which also doubles as a calendar/address book/to-do list on its own.
Palm's offerings include the Tungsten series, the Zire series, and the m1XX series (like the m105 or m125). Handspring has some cool ones like the Treo series, which are a PDA-cell phone hybrid. Sony makes the Clié series, which is also excellent (the text is actually a bit bigger on the Clié). Almost all Palm-compatible PDA's you can get nowadays have some sort of expansion port, as well.
I recommend you stay away from Pocket PC's, because while they are excellent devices in of themselves, they have extreme compatibility issues with OS X. Palm-based devices are quite easy to sync with a Mac, but Pocket PC's have maybe 1 option.