Many of my formative years were the late 90's and 00's. So these two videos that I've found lately have brought back many memories. Very fun, actually.
Sam Tsui does a Britney Spears Medley. I'm not even a huge fan of her but this is fantastic.
Local Vocal (Nordic A capella Vocal Group) does a 90's Medley. Check the video out on YouTube to see the information bar for a list of the songs that they included.