Serial ports are ubiquitous on modern systems. Other than the recent "legacy-free" motherboards and systems, nearly all modern computers have two serial ports, although one may exist only as a set of header pins on the motherboard.
Nothing/Anything. Before you buy an add-on serial port card, make sure you really need it. Embarrassingly often, we've popped the lid on a system, intending to install a serial port, only to find that there was already a perfectly good one unused. In our defense, those little DB9 connectors are easy to miss in the tangle of cables on the back panel, and if the port is disabled in the BIOS, Windows doesn't report it as present.
So, first make sure you don't already have what you need. If it turns out that you do need to install one, think again. Serial ports consume a lot of resources for the functionality they provide. You may be better off installing an inexpensive A-B serial switch and sharing one port between two devices?for example, an external modem and a Palm cradle.
If you absolutely need to add a serial port, buy one of the inexpensive (~$20) serial port cards commonly sold by computer stores and mass marketers. We've used many different ones, all of which seem to work fine and none of which is noticeably better than any of the others.
Belkin Gold Series cables. Nowadays, the only serial cable you are likely to need is a standard, off-the-shelf modem cable. Belkin stocks a great variety of standard serial cables in various lengths, connector types, and pinouts. Although Belkin makes a less-expensive line of cables, its best grade is worth the few extra dollars it costs (http://www.belkin.com).
For updated recommendations, visit: http://www.hardwareguys.com/picks/serial.html.