If OS X is an option "minicom" works well enough. It's a console program rather than GUI but it's relatively user-friendly. (Less hackish than using cu or screen, anyway. It's very reminiscent of Procomm or other similar DOS terminal programs.) You can easily install it with FINK or MacPorts.

(Minicom is the lowest-common-denominator serial terminal program usually included in Linux distributions.)

For classic MacOS... not to endorse piracy or anything, but you're probably not hurting anybody by using "abandonware" at this point.