Many of my clients have been supplied with LAS and LAZ files that haven't had any ground point return classifications performed on them which makes it nearly impossible generate a contour map or surface model. There's a great open source tool called MCC-LIDAR which performs this function very well but it is a command line app and only compiled for 32-bit systems so the number of points it can handle is somewhat limited. There are a couple commercial products that perform ground classification but they are extremely costly and some even require annual subscriptions.
I've written a GUI version of MCC-LIDAR and compiled it for 64-bit editions of Windows and released it as freeware. It has successfully classified the ground returns on a file with 83+ million points and should have no trouble on larger point clouds/point sets.
You can download a copy here: MCC-LIDAR GUI and see my other apps here: LIDAR Widgets