klystrack is a tracker for making C64, NES, and Amiga-styled chiptunes on a modern platform. It has many features, being able to make the sounds of various other consoles.

(Mostly a copy of features from the main page incoming)
Sound 
    -Additive synthesis with filters (think SID, Atari, NES etc.)
    -Hard sync and ring modulation (like SID)
    -FM synthesis (think Adlib, MSX, OPL2 etc.)
    -Wavetable for samples or custom waveforms to be used in synthesis (think Amiga, SNES)
    -Built-in wave generator and editor
    -Programmable instruments not limited to simple arpeggios
Sequencer 
    -Tracker with free positioning of patterns
    -Pattern transpose
    -Pattern length is not fixed
    -Hard limit of 32 channels but this can be raised (just ask if you need more... sounds crazy, though)
    -Fully themeable, includes half dozen themes
Effects 
    -Stereo chorus
    -SNES-style multitap echo
    -Bit crusher/decimator
Export 
    -Own custom format with a tiny playback library to be used in games, demos etc.
    -Export as .WAV
    -Export each track as separate .WAV's
    -Export wavetable items as .WAV's (use klystrack to create one-cycle waveforms)
Import 
    -Import .WAV's to use as samples
    -Import Protracker and FT2 modules
    -Import C64 SID files (Rob Hubbard player)

Here's a good tutorial for learning klystrack from n00bstar: http://n00bstar.blogspot.ca/p/klystrack-tutorials.html

And the handy manual/reference for klystrack: https://github.com/kometbomb/klystrack/wiki