I have lidar data which I want deploy my users using web viewer.
I will probably hold the data in PostGIS (not sure yet).
Is there good open source/free lidar viewer that I can integrate with OpenLayers site?
There's an Open Source project especially to read and write LiDAR (and any other source of pointcloud data) format. This library, called PDAL is located at http://www.pointcloud.org/
I've seen demo using OpenLayers reading data from a PDAL source, but can't locate it anymore. Your best bet would be to ask on the mailing-list over there.
Boundless just updated their Lidar Tutorial that does use OpenLayers : http://workshops.boundlessgeo.com/tutorial-lidar/