LMMS
LMMS is a free cross-platform software which allows you to produce music with your computer. This covers creating melodies and beats, synthesizing and mixing sounds and arranging samples. You can have fun with your MIDI keyboard and much more – all in a user-friendly and modern interface. Furthermore LMMS comes with many ready-to-use instrument and effect plugins, presets and samples.
Installation
lmms is in the offical Archlinux repository, but you may also need the git "master branch" version for the newest improvements.
Know issues
Where is VeSTige ?
VST support has been fixed for x86_64 systems, but is still missing in the packaged version: FS#41302. Until then, install lmms-git or lmms-vst from the AUR.