I got a small Stanley plane that I REALLY like. It has replacable blades that are SUPER SHARP... Cuts balsa like butter... I also have a basket full of small plastic clamps that I got from Harbor Freight. I use them a LOT when building to hold stuff for alignment or while glue sets. Other important stuff in my kit includes -
Saran Wrap to cover the plans
metal yardsticks
larger bar clamps
small band saw and/or scroll saw
small bench sander
palm sander
Dremel tool (Lipo cordless is EXCELLENT) get some Roto-zip bits for this
Hobbico SLot Machine (if you like flat hinges)
Hollow core door covered w/ Homasote for building table (have several)
collection of small sections of aluminum angle stock
4x9 inch drywall sanding block (Harbor Freight, my favorite tool store)
T-bar sanders of varying lengths (can be made cheaply from aluminum angle stock)
a large collection of lead weights (mostly 1 pound blocks, some 2.5 pound)
millions of T-pins