Thanks a lot Johann ! I am honoured ! 
well, that site is just great, you should be 
Draw tool.
I’d go as far saying if the tune sounds like chiptune and is small in filesize, it can be called chiptune without losing nightsleep.
That said, both have quite hazy definition but thats how its been for a couple decades. If my memory suits me right(instant failure possibility) it was 4-mat who first came with the term “chiptune” when he used only a few bytes long samples in his Amiga-mods.
QuadraSID is apparently alright. There’s also Peach if you’re using a PC.
I use a Macintosh and I’ve been involved in the chiptune scene for a couple of years (I use Nanoloop and LSDJ to create music on my Gameboy).
When working on the computer I use:
Automat
Really excellent plug, it’s got sine, saw, triangle and square (which all sound really nice).
http://blog.alphakanal.de/category/automat
Chip32
Unfortunately Chip32 doesn’t emulate the chip sound very well, in fact it generally sounds pretty poor to me, and you’d be better of just using the draw tool in Renoise or whatever.
http://mac.wareseeker.com/Multimedia-Design/chip32-1.0.zip/317397
Other than that a lot of simple, free synths can be used to create sounds that are kinda like what old-school hardware sounds like. Tal Elek7ro is a good example.
Remember that the chiptune sound is really just music that’s made out of waveforms that are basic, so any VSTi that has an oscillator that can produce sines, saws etc. should be alright.