Many image processing programs (ERDAS IMAGINE®, PCI Geomatica®, ENVI®, IDRISI®, etc.)* can import a variety of image formats, including the GeoTIFF files of Landsat data.
Free Multispec software that is included with the USGS lesson plan Tracking Change Over Time can be used for limited spectral analysis.
* The USGS does NOT endorse or support specific software packages. Any usability questions relating to software packages should be directed to the software vendor.