The #1 most common are chocolates and flowers. They're a sure win. Possibly you could do a little something from the heart. Such as cooking her a candlelight dinner.
The type of gift you buy your girlfriend after a fight will always depend on the fight. If it's something small, flowers or a special dinner should work just fine. If the fight has lasted more than one day, or is about an issue that won't be resolved in the near future, a weekend away can help ease the tension.
Well being a girl I can tell you that it is all ways nice to get jewerly (the really nice kind), but it is also nice to take your girl out for a nice make up meal at a nice restaurant. Buying roses is also a nice touch if she likes flowers.
You shouldn't purchase a gift after every fight. Fights happen so don't feel bad. If you want to get a "sorry gift" flowers are a good way to show that you care for your girlfriend.