In need of a flash for a Nikon D5100?

I don't need the crazy CLS as of yet. The flash I need is more is for more of a hobby, I don't need to be professional yet. I take pictures of things, people, animals, etc. I'm looking into the Nikon SB-400, the Sigma 530 Super Flash for Nikon, and the Polaroid 144-AZ for Nikon. I'm going to help shoot a wedding at the end of the month with my aunts. Which of these would be the best? Is there any other suggestions beside these which are good for an amateur photographer? If so please list then!

The SB-400 is a great choice, but it offers no manual-controls. I wouldn't recommend any after-market flashes except Metz. Be sure to buy some Energizer Lithiums OR 3 or 4 pair of rechargeable AAs. Buy it well before the wedding and practice using it. If the ceilings in the wedding venue are low enough, bounce the flash off of the ceiling instead of just pointing the flash directly at the subjects.

For a wedding?

You need at a minimum a good used Nikon SB-600. They cost just about $200 IF you can find them. They are still a very popular flash even after being replaced by the SB-700

While the Sigma promises to do all you need, ONLY Nikon flash units provide all the features you need to shoot on the move like in a wedding. You need to have a flash that shoots in the iTTL mode and only Nikon Flash units (within your budget) can do that. Look for a good flash diffuser too, so you don't get shinny reflections off the faces of the participants.

If you decide to buy the Sigma, visit a camera shop and do some tests to make sure that it has the control you will need (using power settings less than full power (1/2 to 1/64th power) and full TTL control as well

Make sure that you practice using that flash with your camera until you get a perfect shot each time you press the shutter release.