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

Perhaps you could use some sort of volume pedal with your keytar (?)

The Pocket Piano seems pretty damn interesting. I'll have to look into that further.
Squaresynth is actually what I'm using right now. I love the sound and it's simple controls, but I can't get any of the functions on my keytar to work with it. At least being able to control the volume/gain from my keytar would be nice...

Therapsid looks amazing,  but like $500

Alternatively,  you could get an old c64 and a Mssiah cart or Cynthcart,  or Retroskoi+...  Only Mssiah uses Midi though,  the other ones you actually use the c64 keyboard to play notes.

...that midivampire looks pretty rad..

These look alright, especially the therapsid:
http://twisted-electrons.com/

Also, the MidiVampires and similar give some nice lo-fi, chip-esque sounds.

Oh, and look for some nice cheesy second-hand midi modules. They'd probably fit the bill. Especially something like a Yamaha FB-01.

Probably not exactly what you're looking for... But I always liked the Pocket Piano from Critter and Guitari...  There is a midi version now:  http://www.critterandguitari.com/products/pocket-piano

I always thought it had a sort of "chippy" sound,  and they are a lot of fun!

And a good app to try is "Squaresynth",  I really like the sounds in it.  You can find it by searching under the iPHONE  apps,  but it works on the iPad as well.

Putting together a new project after the other band I was in disbanded, and we're looking to do a chiptune-ish sound with all live instruments (everyone's playing a MIDI instrument except for our bassist right now).
I currently use the Alesis Vortex keytar controller, and in my previous band, I ran it through my iPad with a synth app that worked well for that sound, but I can't seem to get any good chip-esque sounds out of it.

Out of the chiptune apps available on there, most I've tried are either glitchy with my controller, or don't offer the control I want (one doesn't even let me control the volume from my controller...). That, and the fact that running through my iPad has always been finicky, has me looking at physical synthesizers.

I've seen a couple online made by small groups that are specifically for chiptunes, but all that I'm seeing either aren't made anymore, or are way too expensive and have to be made on demand. My bandmates are using their laptops, but I don't own one and can't afford one.

What I'm wondering is if there's any synthesizers on the market right now that might work well for a chip-esque sound if programmed right? Something that can take a MIDI in and a standard 1/4" instrument cable out. Any suggestions?