Wavosaur - a robust, portable audio editor that is an excellent alternative to audacity - also has VST support and is great for setting up things like loop points.
Open MPT or Milky Tracker - trackers - while tracked music's popularity peaked for games in the '90's on the amiga - formats such as XM and IT are still relevant today and are even sometimes still used by large well known companies. The advantage of tracked music formats is that if written efficiently they can have good quality audio at a fraction of the filesize of mp3s - they are also more dynamic in that tracks/channels can be switched on or off, slowed down etc in realtime if you know what you're doing. Someone here made a plugin for construct to load some tracker formats, I haven't had a chance to check it yet, but it's no doubt useful.
Hourglass A free, standalone granular synthesizer. High learning curve, but can be extremely useful for creating very impressive sound effects once you know what you're doing.
Tiled Map editor, very useful if you use tilemaps as you can import them into Construct 2 (not all features supported)
Spartan Pixel editor and procedural tile generator (very useful for pixel textures and things like water) - Donationware