Here's a chiptune version of an earlier song:

http://opengameart.org/content/hold-the-line-lospec-mix

Here are some chiptune tips, if you're trying to replicate the sound of an original NES.

The NES has a total of four channels.  Two of those channels play a square wave, one plays a triangle (this is usually used for the bassline and bass drums), and one is a noise generator, which is good for snare drum sounds.

The NES had 8-bit sound, so if you're trying to sound authentic, you need to lower the resolution of your sound output to allow for only 256 levels.  If you don't do this, you'll lose out on some of the harmonics that give it a more authentic 8-bit feel.

Set the volume envelopes on your synths so that they don't fade in or out.  Generally, you want to immediately go to 100% volume, and immediately cut to 0% on release.  Drum sounds are an exception for this -- I set up a simple linear decay on the noise channel for a snare drum, and it worked well.

Some NES pieces have an interesting vibrato sound in the lead instrument.  This is accomplished by having 2 square wave synths, the second one about 2hz off from the first.

You can accomplish a bass drum sound with the triangle wave that slides down quickly in pitch.

wow awesome pointers!  that is super interesting to know :D

Note that I fixed the bassline one my submission (the notes were wrong, and it was too loud).  I recommend listening to it again.

Bart

Here's a Mega-Man-2-esque drum loop. :)

http://opengameart.org/content/get-equipped-8-bit-drum-loop

Are you guys going to let me sweep through this competition unchallenged?  Here's an 8-bit remix of my airship theme.

http://opengameart.org/content/through-pixelated-clouds-8-bit-airship-remix

Here's a link that has the complete NES color palette a few posts down:

http://gmc.yoyogames.com/index.php?showtopic=215169

The NES split its colors up into sets of 4, and each 8x8 tile could use one of these sets of 4 colors.  For sprites, you were allowed 4 sets of 4 colors, and for the background you were allowed another 4 sets of 4 colors.  So theoretically, your background could consist of a total of 16 colors.

Now, the catch with sprites is that one of the 4 colors you choose had to represent transparency, so in reality, NES sprites were limited to 3 colors total per 8x8 square.  Sprites could not have more than 12 colors in total.

So, all told, at any given time, the NES could display 28 colors on screen simultanously -- 16 in the background, and 12 in the foreground.

Here's another silly chiptune:

http://opengameart.org/content/jump-and-run-8-bit