Which font do you need for your current project? That can be a bit tricky to get an overview of, while working in your favorite DTP or graphics application. A font management tool helps you get an overview of the fonts that are available on your computer and makes it easier to compare them.
Font management applications are rarely free. But if you can make do with an older model, Linotype FontExplorer X ver. 1.2.3 for mac is still free. I use it myself and it works like a charm.