Some people believe it's talking about the transfiguration, but that's not a very sound argument. Some people believe He's referring to the Ascension, which was witnessed by some 500 people (and there were presumably some of those 500 in the Luke 9:27 crowd). I think I've even heard it applied to Pentecost. In any case, it certainly does seem like Jesus is referring to His return in this passage. The people standing there when He said it certainly thought Jesus was talking about the establishment of His kingdom on earth. Anyone claiming deeper insight into what Jesus was saying here is just speculating, so any other meaning we try to draw from this must be considered with humility, knowing that you might be wrong.
Other verses in the gospels, as well as some in the rest of the new testament, indicate that Jesus' followers thought He was going to return with their lifetimes. That is indisputable, and obvious when reading the texts. However, in letters (from Paul and others) that are later, chronologically, they seem to realize that they might be waiting for a long time, and we are told, in Thessalonians for example, to keep working at our jobs and not to become idle.