Magnetic Lasso Switch
If you are working with the Magnetic Lasso tool and need to access the Polygonal Lasso tool, press-and-hold the Option (PC: Alt) key and click. That will automatically switch you to the Polygonal Lasso tool. UPDATE: Dave Cross just ran in and added a variation to this: if you press-and-hold the Option (PC: Alt) key and click-and-drag, it will turn into the regular Lasso tool. Thanks Dave!