Creating DSM and DTM from a LAS using a freeware solution
There's a great tutorial from Wichmann et al. (2012) 1 here.
Using SAGA GIS to process LiDAR data and create nDSM and DTM is a 6-step procedure very easy to follow.
Reference:
1- Wichmann, V.; Conrad, O.; Jochem, A.: LiDAR Point Cloud Processing with SAGA GIS. In: Hamburger Beiträge zur Physischen Geographie und Landschaftsökologie 20, S. 81-90.
LAStools! http://rapidlasso.com/lastools/
Super fast LiDAR processing tools (either standalone or as ArcGIS toolbox). They have a las2dem tool which can create output in any of these formats: BIL, ASC, IMG, FLT, XYZ, DTM, TIF, PNG or JPG.
Enjoy.