One of my favorites is the MEV micro-array data analysis tool. It is simple to use and it has a very large number of features.
Works well for any type of data. You can also load into it data from a file that is in a simple text format:
GENE1, value1, value2 GENE2, value1, value2
Feel free to post your favorite clustering tool.
My favourite clustering software is R. Some people may disagree about the 'easy to use' part but once you get over the initial slope of the learning curve it is very easy and hugely powerful. No other tool can really compare in my opinion.