Unfortunately I think when you mention concert posters a lot of people picture a velvet black light Pink Floyd poster or something equally offensive but one of my current obsessions is band and/or concert posters either vintage or orginial by some of the great print makers our there right now.
One of the best websites for them is Poster Cabaret where I actually picked up this Band of Horses poster as an engagement present for my sister and her husband because they went to see them for their first date.
It looks so good framed and I think it made a pretty good gift if I do say so myself.
The other one from Poster Cabaret I'm loving both because I love the band Stars and because I love the artwork is this one, which is by the artist The Small Stakes - definitely one of my favorites on the website.
One great find that I cannot take credit for is a Bob Dylan poster that came folded in his 1967 Greatest Hits LP that my sister bought for her husband Peter for Christmas one year. She got in eBay and then had it restored because it was worn from being folded up in a record sleeve for so long but it looks perfect now.
One of the great thing about concert posters is they tend to come in really standard sizes so its easy to find frames at Ikea or other big stores to fit them so you can save some money on custom matting and framing.