MPlayer/SMPlayer

Personally I prefer mplayer and smplayer (mplayer's GUI) for video playback. It supports almost any video type, external subtitles as well as DVD playback as long as you have the needed codecs installed as shown previously. To install it type:

sudo apt-get install mplayer smplayer
You will find it under Applications -> Video & Sound -> SMPlayer

VLC Player

Many users prefer the Video Lan Client (VLC) as their multimedia player. VLC supports many formats without need for additional codecs. Simply install it by typing:

sudo apt-get install vlc vlc-plugin-pulse
K9Copy

K9copy is a free open source DVD backup, copying, compression, and authoring utility that requires libdvdcss. For other info, see this or this guide. You can easily create MPEG-2. MPEG-4, or DVD videos with this utility. To install it type:

sudo apt-get install k9copy
DVD::Rip

DVD::Rip is a DVD backup/copy program, written in GTK-based perl, that uses the transcode video/audio processing and conversion tools. To install it type:

sudo apt-get install dvdrip