ADB (short for Android Debug Bridge) is a tool pretty much everyone should have. Unfortunately, it comes bundled as part of the Android SDK and that’s a several hundred megabyte download (not to mention that the download page itself is poorly arranged and difficult to navigate for the average user).
Good news though: The Android SDK is rather modular and able to update itself. This means that you can download the “Platform Tools” package (which contains ADB) individually, provided that you know the URL. Figuring the URL(s) is a bit of a hassle to as they are listed in XML here and here. To save you some trouble:
There you go, less than 5 MB total and nothing to install either (unless you are on Windows – why?)