There were a lot of free outdoor concerts this summer around town but they all pretty much ended at the beginning of this month. Still, you can occasionally find some free concerts at some of the places listed above. Some other good places check out are: Double Door Inn, Evening Muse and Fillmore.
The site below is pretty good about having these and most of the other places listed above posted for the coming weekend by Wed of each week (they also include the price for each concert so you can see right away if it's free or inexpensive):
Charlotte NC events - concerts, theater, festivals, kids events & sporting events
They also have a free events section which often includes free festivals that have live music.