My coolest motorway sighting was a gallardo superleggera which was going off the M1 onto the A42 and it was caught in traffic so I thought quick get the window down so I can hear it. Turned out I would have heard it fine with the window up, so loud and such a nice sound as it went from 50ish to double that in what seemed like an instant - made my day that did
Also seen an SLR but that was being driven by an old bloke who seemed concerned about his fuel economy.