The Shruthi is great at chiptune-ish sounds:
http://mutable-instruments.net/shruthi1
A bit off topic maybe, but the Sonic Potions LXR drum synth is also great for chiptune:
http://www.sonic-potions.com/
Then of course there is the SammichSID/MIDIbox SID, which is not just sounding like a C64, it's basically a c64 synth...
The Meedblip is low-fi enough to sound like a chiptune synth as well.
http://meeblip.com/get-one/
Some people use the small MFB analog synths like the Nanozweg and they sound quite cool indeed:
http://www.mfberlin.de/Produkte/Musikel
werge.html