I always seem to run up a few hundred bucks for extras every time I get N+1. The extras include:
Spare tube (essential)
Patch kit (essential)
Tire "irons" (essential)
Seat bag to hold the above (essential)
"Frame" pump (essential, currently all Topeak Road Morphs)
Multi-tool (nice)
Fenders (essential for rain commutes)
Saddle (Brooks B-17, essential for me, maybe nice for you)
Saddle cover (essential for the Brooks)
Blinky butts (essential)
Spare headlight (nice)
Computer (nice)

No wonder I can't afford a new bike!