!Erdas Imagine
You may think that Erdas Imagine is just Arc with an easier installation.  But it's really quite a bit less than that.  ArcInfo does many things and few people really can figure out all that it does.  Most people don't like it because it makes ugly maps.  Erdas Imagine uses orthophotos to do some remote sensing processing.  
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Erdas Imagine has 5 CD's but you only need to install the first one, we don't have licenses for the rest of the software. 
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Erdas has some functions that work with [ArcGIS] so it's probably a good idea to install it after all the Arc products.