For those who don't know (which means you're not a Doctor Who fan), Elisabeth Sladen played the Doctor's companion Sarah Jane Smith during the late Jon Pertwee and early Tom Baker years, went on to a couple of specials featuring K-9 and her own series, The Sarah Jane Adventures, and then appeared during the David Tennant years in New Who.
One of the favourite companions of the series among the fans, her loss is still felt in the Doctor Who community (and the British tv and film industry, of course). And we miss her very much.