If you like mplayer, you will love SMPlayer and Gnome-Mplayer. Both SMplayer and Gnome-Mplayer are really good MPlayer front-ends and they do work great. SMplayer has long been my favorite multimedia player in Ubuntu and we have even featured SMplayer among the most wanted multimedia apps for Ubuntu.
Linux.com: "MPlayer is not your run-of-the mill video player. It's a multi-platform codec-chewing monster truck of a video player for the connoisseur of video players. It has options galore and has the flexibility to play almost anything under the sun."