I just got a Chuck that I can vape at 6 v or 3.7 v. I haven't used any other 'mods' yet, but I love my Chuck!
Drew at Nhaler is about to start selling his Xhaler 'mod' - he's promised to reveal data no later than Friday.
Some of the other mods I have heard of are the Screwdriver, the GG,the Prodigy, and I'm sure other people can tell you more.
The idea of a 'mod' is it's a battery case that will work with one of the standard atomizers and cartridges lke the 501, 801 or 901. Then it's powered with rechargable Li-Ion batteries that you can purchase at many internet sites fairly cheaply and recharge somewhere between 500 and 1000 times. I'm vaping at 6v with my Chuck using rechargeable CR123 3v batteries (aka RCR123A) that can be bought for $5 or less for each. Lasts a lot longer than a 'battery' that comes from the e-cig manufacturer, and I get better flavor/vapor/hit with the 6volt than I ever got from my standard 901 battery, even 901 manual USB passthru's.