The Zep Boys are possibly the longest running tribute band around, performing since 1986. I can remember seeing them a couple of times in their home town (Adelaide, South Australia) in the early 1990s. They've evolved a bit over the years and have toured the UK as "Led Zeppelin Masters", adding an extra twist by peforming with an orchestra. Their singer Vince is an excellent vocallist. https://www.youtube.com/watch?v=SekNp2zAfKw