Tool to make sure that the feature used by a site collection is already installed: http://code.msdn.microsoft.com/WssAnalyzeFeatures
Completely remove a feature from a site collection: http://code.msdn.microsoft.com/WssRemoveFeatureFrom .this handles the problems when a site collection has a feature that is not installed (or is corrupt on the farm), in that case, stsadm -o deactivate will fail. This tool comes to the rescue in that case.
Emad's Blog 
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