1. Advise (DM/Player, a la Sage Advise, Class Acts, Dungeoncraft, Campaign Workbook)
2. Aids (themed random tables, generic maps, cards, counters, etc)
3. Vendor ads
4. Adversaries (monsters, monster ecologies, generic/flavorful NPCs/gods)
5. Optional rules/crunch (skills, feats, spells, magic...