SMPlayer is built in Qt4 and it uses the MPlayer engine for video playback. It's one of the most powerful applications out there for watching DVDs, and it supports plenty formats like AVI, MKV, MPG, FLV.
One of the great features about SMPlayer is that it has a configuration file in which it saves all the settings, separately for each video, including the remaining time. So if you close it then re-open a movie later, all the settings which you've set for it will be automatically loaded, and the movie will continue from where you left it. This does not apply yet to DVD ISO images though.
It doesn't seem to support DVD menus, but you can choose both language and subtitles using its menus.
SMPlayer also supports icon themes, it allows to change the style used, the Qt interface can be configured separately from the settings you have in qtconfig-qt4, it shows detailed information about the currently playing file, it allows you to change default keyboard shortcuts, and you can change the video output driver. You can choose from various video output drivers, the default one being xv.
The last version is 0.6.3 for Linux. Debian Lenny comes with SMPlayer 0.6.1, while Ubuntu Hardy Heron with 0.6.0. To install it, use:
On Debian, type as root:
apt-get install smplayer
On Ubuntu, use:
sudo apt-get install smplayer
To compile the last release of SMPlayer in Debian (this should work on Ubuntu too), download the source tarball from here, uncompress it, change the current working directory to smplayer-0.6.3 (or whatever version you may have) and issue the commands:
make QMAKE=/usr/share/qt4/bin/qmake make install
The last one as root.
Also known as the VideoLAN Client, VLC is yet another cross-platform, powerful video player built using the wxWidgets toolkit. The new release, 0.9.4 provides a Qt interface, as well as many interface changes.
VLC plays pretty much any format you feed it with, including DVDs and DVD ISO images. It handles subtitles well and also provides DVD menus. Included in Debian is the 0.8.6h version, but a new release, 0.9.4 is available for download and repositories are available on their official website. The version in the Ubuntu Hardy repositories is 0.8.6e.
The only thing I did not like about VLC is that its interface is somewhat inconsistent, for example the Preferences dialogue has a small size by default, and even if you change it, it will be resized again next time. Also, sometimes the F shortcut for fullscreen does not work.
Kaffeine is a wonderful video player, currently available for KDE 3, but a port for KDE4 is in the works too.
Kaffeine uses the Xine engine for playing videos, and it does a very good job especially with DVDs, reading menus very well. It's light and it starts fast, integrating very good with KDE. The last stable version is 0.8.7, which is included in Debian Lenny repositories. Kaffeine plays pretty much anything and it offers the possibility to configure the Xine engine too, but it uses a little more CPU power than SMPlayer when it comes to high-resolution videos, like x264 Matroska videos of 720p or 1080p.
As I already mentioned, it handles well any type of format, including DVDs, DVD ISO images and mounted images. However I noticed that sometimes it won't play a DVD ISO image, but after mounting that image and pointing Kaffeine to the directory where it was mounted, it won't have any problems playing it.
Those three players listed above are the ones which work great for me, and I can play any video content using them. There are tens of other video players out there. I could list just a few here: KPlayer, Codeine, Dragon Player, Xine-UI, Totem, GXine, RealPlayer and so on.