I typically use Windows-based FUSION to generate grid metrics, which can be used to describe forest structural characteristics. From the FUSION documentation:
GridMetrics computes a series of descriptive statistics for a LIDAR data set. Output is a raster (grid) represented in database form with each record corresponding to a single grid cell.
FUSION and RSAC have excellent documentation and tutorials--for example, page 5 of EXERCISE 5: CALCULATE LIDAR METRICS highlights how to use FUSION to calculate grid metrics such as: count, skewness, cover, 75th percentile value for cell, etc... More details on grid metrics can be found on page 69 of the FUSION documentation.