Kaffeine - Kaffeine is a video player for KDE3 (work is in progress at a port for KDE4 too) which uses the Xine engine and can play pretty much any video format. Last version is 0.8.7.
Amarok - The most popular audio player for Linux, Amarok features everything one can ask from an audio player, from powerful collection management, to support for devices, lyrics fetching, scripts and many more. Heavy work is put in the Amarok 2 for KDE 4 port, which will feature among others a completely new interface and widgets.
JuK - Another audio player for KDE, JuK offers only basic features, but it's lighter and faster than Amarok.
kdetv - Although I prefer TVTime, kdetv is a nice TV viewer for KDE3 providing many configuration options. The official website was down at the time of writing, but you can download it via kde-apps.org, here (try here also). Also, I'm not sure but development since to have stopped.
Noatun - KDE audio and video player based on aRts. Noatun is included in the kdemultimedia package.
KsCD - the KDE audio CD player, KsCD also allows CDDB fetching of song information.
Codeine - this is a small video player for KDE3, with support for playing DVDs and VCDs too, along with usual video formats. The port for KDE4 is called Dragon Player.
KMPlayer - Powerful KDE video player which allows you to choose which backend you want to use, like Xine, GStreamer, MPlayer.
KPlayer - KPlayer is yet another complete KDE video player, which can also play TV if you have a TV Tunner card.
SMPlayer - SMPlayer is not exactly a KDE application since it doesn't use KDE libraries, but it's built in Qt 4. It uses the MPlayer engine, and overall is one of the most powerful video players I've used. Also, I noticed MPlayer performs better than Xine (considering CPU resources), at least when playing high resolution movies.