I agree with the fish Sarah mentioned except for the dwarf angel, they need a bigger tank than yours at least a 70 ga. Go to
www.liveaquaria.com and check the fish list they have there. I also agree that you should stick with the semi-aggressive ones.
Just take this little tips in count to do your stocking list:
You shouldn't have more than two clowns in a tank that big. I'd recommend you to get another maroon if you want to have a pair of clowns or maybe a black one or an ocellaris. Do not choose a cinnamon, a tomato or a goby clowns because those are from a different species that the one you have, and they will probably fight to death for territory. Also if you do choose to get another clown, do not choose one that is bigger than the one you have, choose a smaller one that can be the male. If you are lucky they will become a mated pair.
Wrasses are beautiful fish, very colorful but, you will need to put a net or a screen at the top of your tank because they like to jump out of it. This way you avoid finding them on the floor some day.
If you don't have a deep sand bed avoid fish that require one to be happy like most goby fish.
Stay away from damsels, they are very aggressive fish especially the domino's one. I read a thread here from someone that have two in his tank and lost three of his fish to them.
Choose fish that are reef safe if you're planning to have a reef tank.
If you want to keep shrimps be careful, don't choose fish that will eat them. Dottybacks have a taste for shrimps because they are they natural pray. I didn't know this when I bought mine.
These are all the things I can think of now. Let me know if you have more questions, feel free of PM (Private Message) me.
Have fun doing that stocking list. ;)