-Dual lands, especially of the blue variety.  You may never need a Plateau, but a set of Volcanic Islands is handy to have on hand.
-Fetchlands, it may be that just one play set is enough.
-Restricted goodness.  Don't worry about getting 1 of every restricted card.  Odds are, you'll never need a Mind Over Matter or Dream Halls, but some cards, such as Demonic Tutor, Balance, Yawgmoth's Will, Tinker, and so on will fit into almost any deck of the appropriate color that you build.
-Artifact acceleration.  Even if you can't afford Moxen, cards like Sol Ring, Mana Crypt (if it's in your price range), etc are very useful to have on hand.
-Common/uncommon staples.  Shouldn't be too hard to get, but stuff like Brainstorm, Dark Ritual, Duress, etc as well as sideboard staples like Tormod's Crypt, Red/Blue Elemental Blasts, Naturalize/Disenchant and so forth are great to have.
-4 Force of Will, and to a lesser extent a few Misdirections.
-4 Wastelands, 1 Strip Mine
-Null Rods, although following some of the members forum discussions, these may become less necessary.
-Goblin Welders if you have any intention of playing Workshop-style decks
I'm sure I've left off a ton of stuff, but I thought I'd at least get started 
