is there any free open source tools available for viewing TIN (triangular irregular networks) files( which i got from LAS (Lidar data) file...)?
thanks!
This will depend greatly on the format. Most TINs generated from LIDAR data will be in one of the standard GIS formats, in which case a good, open source GIS program will be your best bet. Any of the packages which use the GDAL/OGR libraries will most likely be able to handle the TIN format you're using (whether that's ArcInfo's, etc).
I'd recommend trying either QGIS (either to learn) or GRASS (supports the most options/formats).