Mac: There are plenty of browser tools out there for testing your internet speed, but they're usually filled with obnoxious ads. If you'd prefer a native app, Speedster is free and does its job well.
With the free version of Speedster you get three tests a day, which should be plenty. (There's a one-off in-app purchase that grants you unlimited tests.) Just launch the app, click the test button, and it will do the rest. It's a little thing, but it's still pretty great to keep around for when you're worried about issues with your Internet speed.
Speedster (Free) [Mac App Store]