I do all of my composition on Linux. The only non-open source stuff I use is renoise (it's my main daw).

Here's some stuff I've used

• Qtractor, a decent piano roll sequencer: http://qtractor.sourceforge.net/qtractor-index.html I'll use this when I want to export a MIDI from something I've written in renoise

• Seq24, another piano roll sequencer, but focused on creating smaller loops that you build a song from http://www.filter24.org/seq24/ Never wrote anything in here, but I actually use this to practice instruments by making little loops I play along with

• Alib Tracker, a tracker of OLP3 http://www.adlibtracker.net/ I've written one song in this, pretty good

Plugins:

• Calf plugins, bunch of stuff, I use them to master http://calf-studio-gear.org/

• Hexter, DX7 emulator: http://dssi.sourceforge.net/hexter.html

• Qsynth, SF2 module: http://qsynth.sourceforge.net/qsynth-index.html

• Whysynth, really powerful synth, my go-to if I need some synth noises: http://smbolton.com/whysynth.html

• JAMin, a tool for mastering http://jamin.sourceforge.net/en/about.html I used to use it but I've since switched to calf tools for everything

Drums:

• Hydrogen, a drum machine http://www.hydrogen-music.org/hcms/ I've never written music in it, but I'll use it like a module, I'll pipe midi into it to use their sample packs, which can be pretty good

All of the above mostly use Jack, which I recommend: http://www.jackaudio.org/
I use qjackctl to connect all my jack devices: http://qjackctl.sourceforge.net/