Thanks to the work of two Googlers, there are now direct links to the approximately 3.5MB platform-tools folder that includes adb, fastboot, and several other utilities.

You can get the platform tools for Windows, Linux, or Mac direct from Google's servers. You get a ZIP file containing the platform tools folder with ADB, fastboot, and a few other tools.

Having ADB and fastboot available outside of the other two main downloads, first of all, saves space. Android SDK takes up 292 MB of space.