At uni I used ERDAS imagine to view two rasters at the same time and it had various viewers for comparing them. I'm looking for something that has a slider so that I can see the differences between two Landsat images (same area different dates) or if not a slider some good functionality for visually comparing them. ArcMap is too slow at switching between the two. Is there anything free that I could use for this?
I have not tried it but Opticks is worth a shot: http://opticks.org/confluence/display/opticks/Welcome+To+Opticks
Actually, I have had some good experience displaying images with QGIS. And you can play with transparency. But from what I know you cannot have two linked viewers.
ENVI also has a nice feature for geographically linking two viewers, but it is not free. Idrisi has no linked viewer either from what I know (But the last vesrion I've worked with was Kilimanjaro)
Note that the key for displaying large images is the presence of pyramids, and the use of square tiling when they are very large. You can use gdal for that.
I recently discovered an ArcMap answer to my question:
The image analysis window has swipe tool which is exactly the tool I was after.