My favorite tool is probably NoteMan from MartinScott. If I have to pick one tool from the suite, it would be either NoteMan.Editor or NoteMan.DocDelta. It is very hard to decide between the two of them. Editor is great for editing documents, see the contents of different fields and even change data types. I use it to get the UniversalID of documents and much more. DocDelta help me solve replication conflicts quickly and easy. I can higly recommend the NoteMan suite of tools to any Notes/Docmino developer, and for the price ($395 for the whole suite), you get a lot of functionality.
From TeamStudio I use Undo (previously called Snapper) to make snapshots of the design while developing for easy roll-back, Profiler to find performance issues in my code and Configurator for search-and-replace through-out a database (design and/or documents). Those tools run around $500 each, if I recall correctly. I also use their free class browser, a tool I highly recommend to everyone doing object oriented Lotusscript development.
From Ytria I use a number of tools.The two I use the most are scanEZ and actionBarEZ. The latter is great when I want to apply a specific design of action bars to many forms and/or views. I design the action bar in one view, with colors, fonts, backgrounds, etc. When I am satisfied I can update all views and forms the the database with the new design. I don’t use scanEZ as much, but still on a regular basis. It also have functions to identify replication conflicts, like NoteMan.DocDelta, but the two tools complement each other. Using scanEZ, I can locate and delete documents of a particular type, including profile documents, and much more. I also sometimes use designPropEZ to check the design of a database and make sure it does not inherit element from the wrong templates/databases.
Here is a screenshot of my currect toolbar with all my development tools: