Depending on how much design work you've done, and what kind of software you're using, you probably have some idea of which fonts you have installed and what they look like. If you don't, there's a wonderfully simple Web app called Flipping Typical that gives you a WYSIWYG heads up of all of them at once.
You can type in whatever text you want to see and it will show you how it looks in each font, and the site updates in real time to reflect the changes. You can also use keyboard shortcuts to toggle on bold or italics to see what it would look like with a hint of formatting. It's smart enough to remember what font you were looking at between sessions, so if you close the page and come back later it will come back with that formatting still intact.
Most modern Web apps with font control have previews you can see before changing a font, but this app differs in showing what you have that can be used in offline programs too. I like it because it's good for brainstorming a quick font choice, something which can become increasingly difficult the larger your font library gets. For business cards, fliers, posters, and more, this can be a simple way to cut development time down.