Does anyone know if they make a 16x2 LCD that runs on 3.3V and has a I2C interface? I could really use one but haven't found one yet.
Use a PCF8574 I2C backpack (the normal kind you see on ebay and shit) - they work from 2.5v to 6v - on a 3.3v rated LCD.
IME, the 1602 LCDs I've tried all work at 3.3v. You can buy ones specifically spec'ed for 3.3v from buydisplay.com , though (just be sure to get one with the 1x16 header along the long edge, not the 2x8 header), if you need to be sure the one you get will in fact work. That site has a nice selection of character LCDs
Edit: Nope, I found that most of the "5v" LCD's don't quite work at 3.3v
DrAzzy:
IME, the 1602 LCDs I've tried all work at 3.3v. You can buy ones specifically spec'ed for 3.3v from buydisplay.com , though (just be sure to get one with the 1x16 header along the long edge, not the 2x8 header), if you need to be sure the one you get will in fact work.
Hang on! You need to be clearer there! 
You mean to say all the 1602 displays designed for 3.3 V will work on 3.3 V.
Clearly those marketed for 5 V will not as they require 4.5 or so Volts for the contrast voltage and do not have the charge pump on the 3.3 V version required to provide that contrast voltage. The HD44780/ clone chip itself does not care either way - so it will work from 3.3 V if you provide another negative source for "Vo" - pin 3.
What's wrong with the 2x8 header ones?
Paul__B:
The HD44780/ clone chip itself does not care either way - so it will work from 3.3 V if you provide another negative source for "Vo" - pin 3.
I'm not following this last statement.
I have not tried to use a I2C backpack on the LCD running at 3.3V. I looked up the datasheet and see the PCF8574 should work fine at 3.3V.
I have use a 3.3V LCDs in the past, but in parallel only (not I2C). I don't recall doing anything special expect for ordering a 3.3V unit.
I'm 99% sure that I've used random 5v 1602/2004 LCDs with 3.3v.... you have to adjust the contrast higher, but I was still able to make them work fine...I'll test this tonight if I remember, as I have something that I can easily test with.
The problem with the 2x8 header is that you need to make a cable to convert from the 1x16 header on the backpack to the 2x8 header - so you no longer have a backpack, but a dongle dangling from dupont jumpers.
I just did some testing - you're right, the 5v screens don't quiiite work at 3.3. You can read them at max contrast, but just barely.
Yes, the optimum contrast voltage is usually about 4.5 V. If this is set up with 5 V from USB and the USB starts to sag as it seems to on a module that has been sitting and running permanently on my shelf for a few years (it is also running a dot around a couple of MAX7219 matrices), it gets a bit dodgy - I estimate the contrast voltage to be about 4 V at that point, so 3.3 V would be pretty sad.
I think when I get home I will have a play with that old piece. Not sure why it "sags". For a start I will perform my own recommended modification and break the connection to 5 V on the contrast potentiometer. 