Here is my list useful tools which have helped me in various activities involved in developing software. Most of which are indespensible :)
Visual studio - can’t do any windows development without this. Visual studio premium provides really good set of tools, but the free community edition is mostly sufficient for development.
Eclipse - Really essential tool for Java / Android development for desktop / web, it supports lot of plugins and Rich client application framework. You can customise it to do any language development even ARM.
Visual studio code - this is the latest addition into my toolbox, and microsoft have done it. It’s absolutely perfect for web work / scripting quick editing.
Scite - Before visual studio code I was using this scintilla based editor as a lightweight but very powerful ruby / scripting editor
CodeMaid - Really handy free tool for cleaning up the .NET codebase. Supports batch operations as well. Supports most important customizations, could be better but still very essential.
Resharper - Probably the best developer tool (addin for Visual studio) for .NET development. It really pampers the developers, once hooked difficult to do without.
nCrunch - Concurrent test running tool. At first I thought why would I need it but, its really clever and very useful while doing test driven development. It gives a live line by line code coverage and test result while typing… imagine that.
specflow - Behaviour driven development framework and tool for .NET development in visual studio.
Mercurial - An alternative to Git, Tortoise Hg is a great tool for this - I was using it before GUI tools for git were not present on Windows.
Diffuse - good opensource python gtk based diff tool, it shows word differences, lightweight does what is supposed to and gets out of the way…
P4Merge - its a free tool from perforce, I only use it for image comparisons…
Gimp - Fantastic tool, great effort from opensource community. This sorts my most of the raster image editing needs, I didn’t find any need for photoshop. Its got a great scripting interface Script Fu for batch editing or procedural editing.
Inkscape - Again, another opensource winner for vector graphics designing
Dia - Good free tool for doing illustrations / structured diagrams with connected elements with lot of clip art like things
Microsoft Viso - This is hands down the best diagramming tool money can buy with very big library of drawable elements. Outputs in various formats, also supports printer friendly versions. Good for professional documents.
Powershell - Useful for automation of tasks. Powershell scripting can save a lot of time, it can interact with .NET assemblies as well
Ruby interpreter - Interactive ruby prompt is also very handy for everyday automation, scripting, file manipulation, operations on strings etc..
OneNote - Handy notebook tool which works across devices, supports clipping from internet (html copy paste), drawing etc..
Focus booster - Pomodoro technique is a great tool for improving productivity, I found this to be a great application for desktops, its free other features are not free.
Kuler - Adobe’s kuler which is perfect tool for getting colour palettes for websites / applications / designing work
TuneIn radio - Online radio to listen to while working hard, focus booster and this is a paradox :)
Microsoft Excel - Spreadsheet tool with wide applications, once if you get used to it, its very difficult to do without. The online version is free but doesn’t support scripts.
Openoffice Calc - A free alternative to Excel, not that powerful but does the job…
Task scheduler - the good old windows task scheduler can automate a lot of things, really easy and handy tool.
This maybe required for 3D printing, art work or game development or illustrations / walkthroughs etc.
Blender - I am a power user of blender, have been using it since the begining of it, its an inspirational opensource project. Nothing else is required for doing 3D. A 3D modeller, renderer, game and physics engine, it also supports video editing… phew. STL is supported for 3D printing. Great extensibility via the python scripting interface.
Rhino3D - Advanced tool for NURBS and surface based modelling. Not a free tool, it supports plugins.
TinkerCAD TinkerCAD is a free online CAD tool. Its not having very elaborate tools but mostly gets everything done. Good for hobby 3D printing.