The Bitsy Companion Board with Ethernet option has an Ethernet controller that supports 10/100BT. You can also use an NE2000-compatible PCMCIA or CF card that supports 100BT (eg. the Hawking CF686TX 10/100).
Note: Due to limitations in the StrongARM architecture, you will probably not notice a performance difference between 10 and 100BT Ethernet adapters. ADS only recommends 100BT cards if you are connecting to a 100BT-only network. (January 8, 2002)