When I first started GMing, I honestly didn't know what tools were available to me to make my weekend masterminding easier. But in the years since, I've become inordinately dependent on a few key ingredients that can serve any GM, no matter the game. These are all cheap and inexpensive items I wish someone had suggested to me early on.
* At least two of the D&D 7 dice sets - This naturally depends on what system you're using but having at least 2X percentile dice, d20s, d12s, d10s, d8s, d6s and d4s insures that you have enough to cover your rolls and quite possibly someone who forgot their dice (or can't afford any). 7 dice sets are cheap, anywhere from $5 to $10 at the most.
You might also use any of the hundreds of free dice apps that exist on Google Play or Apple's App Store. It just won't look as pretty.
* A Chessex vinyl mat & a set of Expo Vis-A-Vis wet erase markers - Quite possibly the coolest arrow in your GM quiver, next to your imagination, Chessex mats come in three sizes, the lowest end as cheap as $20. Best used with Expo's 8 colored wet erase markers ($7), these babies are perfect for mapping out game environments, be they labyrinthine dungeons, treacherous swamps, post apocalyptic Thunderdomes or the sleek corridors of a sci-fi starship. Use them with painted miniatures or simply quick-draw character positions in various terrain.
* Officemate Slim Clipboard Storage Box - Most players won't spend much time or money on safeguarding their character sheet. And yet a game is paralyzed without everyone on the same page. Unless you want to store their character specs in a more digital format, these plastic clipboards store nearly 1.5 inches of papers or flat items and can hold several more in their metal clips. AND they come complete with a pencil tray for character sheet updating. These are a bit pricey (sometimes $13 - $15 each) but extremely useful for players of any game.
* Auditory Atmosphere - Need to immerse your players in the eerie tones of a dungeon environment? MyNoise.net is a great resource for setting ambiance. They've recently released their Dark World soundscape, playable via their site or their iPhone app. Give their Animate button a whirl and keep it dynamic.
* Obsidian Portal - A veritable one-stop-shop for GM scheming. I couldn't run my games at all without this site. A customizable RPG wiki, OP is a boon for GMs who need to keep track of all their PC bio & crunch as well as NPC info and stats, game ideas and secrets (player accessible only at a GM's discretion), adventure logs, calendar announcements. The list goes on and on. The site is free but for $3.99 a month, the Ascendent membership grants you far more expanded features (multiple map storage space, a forum, email notification ability and unlimited campaign-making).
* Eureka: 501 Adventure Plots to Inspire Game Masters - Stuck coming up with a plot idea for your upcoming game? Worry not. This $17 PDF is chock-full of story seeds and suggestions, be it high fantasy, sci fi or modern settings. Five hundred and one suggestions, as a matter of fact.