Too many to list in this post!
I have signed up for quite a few and taken advantage of their sign up bonuses.
For the most part, they are all paid off but the ones that aren't are still under a certain "no interest" period promotion.
Airline cards can be a great way to save some money on travel. For example, last year my wife got the AA card and to meet the spend requirement, we just put our rent on it for a month and then paid it off right away.
Doing this gave us 60,000 AA points which actually ended up getting us round trip tickets from Chicago to Honolulu, Hawaii which we used to finish off our 50 State trip we started last September.
Of course, you do have to be very careful or credit cards can end up costing you more than the benefit, but if you handle them correctly, they can actually be really useful!