These are tools that I have used when modding the Mass Effect games. They are not all modding specific, but you may find them useful.
Mod Manager allows you to easily install and uninstall mods as well as make new ones.
ME3Explorer is a toolset for Mass Effect 1 - 3. The ME3Tweaks Fork contains bleeding edge features.
Mass Effect Modder is a texturing tool built by Aquadran. Designed for speed and engine accuracy.
Sublime Text is beautiful, fast, and powerful. Swanky features make this a mainstay text editor.