Seeking open source software package for Remote Sensing?

There are a few good ones around:

• Orfeo Toolbox
• GRASS
• SAGA GIS

All with the bonus of being able to be used though the QGIS interface using the SEXTANTE plugin like so http://blog.orfeo-toolbox.org/uncategorized/otb-inside-sextante-inside-qgis

For processing of Landsat, I can recommend GRASS. I tried many others.

You may need to refine your question with regard to the type of imagery you propose to use. There are workflows which have been more or less developed and implemented in various software.

Not only the type of imagery, but the purpose of the processing and final analysis. For Landsat, I am interested in a quantitative value. Which is different to qualititive methods used in regional classification of vegetation for instance, methods and tools for this work are more common.

You will not likely find a Swiss Army Knife for free. But you will find very specialised tools which do one job well.

R is also suitable as a GIS. Many of the standard GIS functionality is available in pure R, e.g. interpolation (gstat, automap, fields), raster operations (raster, sp), or polygon operations (rgeos). In addition, many of the statistical techniques (e.g. regression, PCA, classification), can be used also for spatial data and are readily available in R. For any missing stuff, you can interface R with GRASS and SAGA. See the spatial data task view for R for a good list of spatial data analysis in R.

Ofcourse, R is a programming language which has a rather steep learning curve, especially when you are used to GUI based GIS software. However, in return for your investment you get a statistical environment in which you can do just about anything out-of-the-box, or create it yourself if it is not already available in a package. Also, in comparison to GUI based software you can easily script your analyses, making them easy to repeat, and version control.