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