Ammonite is an add-on to DEVONtechnologies' excellent product DEVONthink Pro.
While DEVONthink allows you to tag your data, it's search capabilities are currently very limited when it comes to tags. There is no way to retrieve documents from DEVONthink which are not tagged with a certain tag. Searches for data tagged with multiple tags are possible, but involve opening the find panel, clicking 'Advanced…' and building the query manually. You have to type in the tag names and you are not given any clues about available tags.
This is where Ammonite comes in.
Ammonite visualizes the tags used in a database and finds documents by including or excluding them from your search. Tags applied to many documents will appear larger then tags applied to a few documents. This helps you to drill down on the information you are looking for.
You can limit the tags being displayed with the tag filter field in real time.
Ammonite is easy to use: Left-click on a tag in the cloud to find documents having this tag applied. Right-click to exclude documents with this tag from the result. Combine as many tags in your search as you need. The result is updated immediately. That's all there is to it.
Whenever a tag is included or excluded from the search, the matching documents will be displayed. Details about the selected document will be shown as well. From here you can reveal or open them in DEVONthink, Finder or an external application.