Best flash for nikon d3100?

I have recently bought a nikon d3100 and after receiving some christmas money i thought i might want to invest in a flash.
I have two questions:
Which flash do you recommend for this camera?
and is it possible for me to you flashes not made by nikon ( like the cheaper ones on ebay) on my camera?

Price really is an issue for me as i have a very small budget, being a student.

The SB-600 is a great speed light at an affordable price, around $220.

Anyone on a small budget shouldn't ask for best flash. Nissin and metz make iTTL compatible flashes. Look for something where the head can move up and down as well as rotate. Nikon's sb-600 is a good safe choice.