I recently bought MiniBit from AudioThing, which is an 8 and 16-bit synthesizer. At first I wasn't really sure if it would fit my needs, but I have to admint, for 20$ you get something really cool! I actually almost used it exclusively on my first released track (you'll find it under Constructive Criticism, I posted it there 
).
Of course, there are tons of very good free plugins and trackers, but since I'm coming from a DAW background, MiniBit allowed me to make chip sounds but the tweak them with modern effects.
All in all, I highly recommend the software, if you're working with plugins and want to invest a couple of bucks. Believe me, it's worth it.