Black & Bleu
Comprehensive list of Mac OS error codes with clear explanations

GURU 2.9
The late Newer Technology's Guide To Ram Upgrades

SpellTools
Universal spelling checker, works in any application. Includes updater for use in Mac OS 9

System Errors
Simple and slightly dated, but very easy to use list of Mac OS errors and meanings

VisualFont
browse libraries of TrueType and bitmapped fonts