There are many qualities that come to mind. The first must-have for a good friend is honesty. While we all like a little ego boost every now and then, most of us don't want to be lied to in the process. I would much rather a friend tell me that my butt looks enormous in a pair of jeans than to secretly think that while telling me I look great.
Another aspect of honesty in a friendship is for the friends to be honest with each other. We all have good times and bad, and we have all done things we aren't proud of. True friends don't lie to make themselves look better. In fact, it's the honesty that looks pretty darn good to me - not what you have or don't have, not your social status or your bank account. Some of my best friends have very few material possessions, but they would drop everything for me if I needed them. That is what matters to me.
The next quality a good friend should have is trustworthiness. I cannot stand gossip. As a result, I have always tended to have more male friends than female friends. I don't necessarily think that most women gossip. It's just that men don't often talk to others about other peoples' feelings and secrets. I know I can trust the females in my life who are friends, even the ones I've made on this site. I don't expect never to have arguments or disagreements with my friends. I just expect them to keep the private things we've shared between us.
A good friend should have the ability and willingness to listen as well as share. I've had "friends" who only want to talk about themselves and share their successes and failures. They find ways to steer every conversation back to themselves. I'm the first to listen when a friend needs me, but I also want to be sure that the friend will be there as well. I've had too many 'fair weather friends' in my life, so one who doesn't dominate every conversation is a must.
Finally, a good friend has empathy. I'm there to laugh with my friends as well as cry with them. And let's face it, a good friend can tell when you're having a bad day. That friend should offer comfort and encouragement or even a swift kick in the butt if I'm fretting over something silly. If I'm particularly proud of or happy about something, that friend should be pleased for me - not resentful or jealous.
A lot of the qualities of a good friend are immeasurable, and it's impossible to list, or even think of, them all. But I think the qualities I have listed are a necessity for a good friend.