Weird crap... there's a lot of strange stuff out there. I'm a bit of a networking geek, so I might choose to add as many different network protocols I could find. Ethernet is a no-brainer, but there's so much more out there. Easily available in PCI are things like token ring (Madge or Olicom cards), 100VG (HP, conveniently also backwards compatible with 10MbE, and featuring the most brain-dead QoS system out there) and FDDI. WiFi is also possible, although Win98 drivers are so primitive that interop with modern (WPA2-secured) networks isn't really an option. But there's more, although finding these in PCI form factor could be a challenge - things like ARCnet, ATM, LocalTalk etc.
Of course, then you have the challenge of getting them something to talk to 😉