[This article is obsolete, but the serial cable is not, as it turned out. For more info, read about the Yost cable standard.]
Pretty soon, serial cable headaches will be a thing of the past (if USB finally takes over), and serial cables will once again be the province of patient admins poring over stacks of printouts and diagrams.
Back in the day, though, every hobbyist had to custom make thier own serial and parallel cables. (Hobbyists, flush with confidence having built cables would attempt soldered-in RAM upgrades, and burn out expensive 64K RAM chips.)
If you need to interface betweeen a modular serial port (usually in the wall at an older installation) and your computer, I like using 9 and 25 pin to modular adapters with the connectors you can swap into the proper holes. You can use a plain cat-5 cable to connect all the parts. (Cat 5 cable has weird crossovers, but the net effect is that it's wired straight through.)
Before proceeding, it's a good idea to search "Google" with a search phrase like "VT220 modular serial cable". Include the name of the terminal being used at the site.
These modular cables are great substitutes for serial cables - you buy a pair of modular ends, and wire with cheap cat 5 in between. The cables work out to just over $15 per interconnect, but are completely reconfigurable to work with just about any kind of cabling conundrum.
You can even build null modems with these ends.
I haven't used these guys, but this place has cheap parts http://www.cablesnmor.com/modular.html
Here's a page that defines a complete set of these cable ends. It's the above idea fully developed into a really useful product:
And here's an alternative method that's pretty darn clever: