Can I make a sine wave with the arduino

I would like to use the arduino to make a sine wave generator. Would it be capable of making a nice smooth one?

No. You need an external D/A to get anything close. Why does it have to be so smooth?

You cannot with the Arduino alone but you can get near with a bit of additional hardware and some clever programming (as long as the frequencies are not too high):

http://interface.khm.de/index.php/lab/experiments/arduino-dds-sinewave-generator/

Make a sine wave is a broad term!, I assume its a sinusoidal voltage you want to create?, it depends what you want to do with it

I managed to build an inverter with three Uno’s which gave out a sinusoidal (ish) current but it wasn’t smooth

A software based sine wave is pretty easy with an interrupt and lookup table and a software based sine is good for lots of applications

pylon: You cannot with the Arduino alone but you can get near with a bit of additional hardware and some clever programming (as long as the frequencies are not too high):

http://interface.khm.de/index.php/lab/experiments/arduino-dds-sinewave-generator/

Wow, that is clever. Am I reading this right - DDS in principle is just PWM plus a smoothing filter? So the Arduino can do DDS up to some fairly low frequencies only because it can only PWM so fast, but dedicated DAC chips do it in principle the same way, just faster since they are implementing PWM in hardware and not as software inside a microcontroller?

DDS in principle is just PWM plus a smoothing filter?

Not quite PWM but close enough.

but dedicated DAC chips do it in principle the same way, just faster since they are implementing PWM in hardware and not as software inside a microcontroller?

No totally wrong. See:- http://en.wikipedia.org/wiki/Digital-to-analog_converter