A used IBM Thinkpad, model 760XL laptop (P166, 104 meg RAM) running Red Hat 7.1/7.3. Sure, it's slow and has a funky keyboard, but it beats the heck out of a No. 2 pencil and a Big Chief tablet.
Update: upgraded to a 770Z Thinkpad (P2-366, 192 meg RAM) running FC3. Anyone feel like donating a later-model laptop to a starving writer <g>?
Update: upgraded to a A31 Thinkpad (P4-1.6, 512 meg RAM) running FC8. No longer starving, and no longer soliciting donations <g>.
Bram Moolenaar's powerful SGML-aware vim text editor.
OpenJade, a DSSSL rendering engine for converting SGML documents into other formats.
DocBook, The Definitive Guide, by Norman Walsh and Leonard Muellner (O'Reilly, ISBN 1-56592-580-7). This is still the standard reference for anyone attempting to write a document in Docbook SGML format.