Great question. And great responses.
Like they say, depends on what you want to make, and what materials you want to use. That said, for me it would be: Dremel & bits/tools, files, razor saw, hack saw, ruler, calipers, compass, sandpapers, primers, paints, safety glasses, clamps, rubber bands, masking tape, respirator and dust masks, X-Actos and different blades, hot glue gun, eventually a heat gun. Plus lots of references (pics and patterns/blueprints where available), a computer and an internet connection, printer, DVD (or better yet, Blu-Ray) player and discs.
For what I do, besides various glues (Elmers, superglue, epoxy, wood glue), and spot putty (I usually use automotive red spot putty) I also find Plumber's Epoxy Putty and Minwax Wood Hardener occasionally priceless. Menard's has a Plumber's Epoxy Putty in their plumbing section, but there's a better one (I forget the brand name) in their paint and glue section. It cures quickly (5 minutes or so), and gets very hard. It can be used for attaching pieces, and for creating shapes to join other shapes. Be prepared to file and sand, then spot putty and sand... Apoxy Sculpt is another great epoxy putty. More expensive, but you have a much longer work time; you can actually sculpt it, and need less spot putty and sanding. Minwax Wood Hardener is great for taking wood, MDF and cardboard and making it hard and waterproof(ish). Really handy when working on some of the thicker cardboards like chipboard (super thick cardboard). You do not want to use that stuff without good ventilation.