Quick and easy ways to check SQL/DB performance We host a web application for customers, sometimes they call in and complain of slowness. I was asked to have a list of things to check from network down to the application. 

For SQL, can any suggest any quick queries that can be run or any good commercial tools that could be purchased that would quickly determine if SQL is the root cause or is otherwise healthy?
The Brent Ozar Unlimited First Responder Kit is a good and free place to start. It will walk you through looking for slow queries and comes with a handy worksheet of what to do.