Outside of a screw driver and standard wrench, one of the tools I use incredibly often are hex wrenches. I needed a single hex wrench about a year or two ago, but I wasn't sure what size I needed. So I went to Home Depot and bought a 13 piece set and I am so happy I did. I use it for a ton of things, but mostly for tightening the screws on furniture, chairs and other misc items throughout the house and office.

Here is a picture of it:
Husky Hex Tools

The thing on the right is an arm that can be attached so you have more of a grip on tightening screws, if necessary. Here is a taller image, so you can see what I mean. You basically pick the hex wrench, pull out that arm thing and plug the hex wrench into it. Works well...

Husky Hex Tools