MooPlayer is a free, open-source media player for audio and video files with some interesting features.
To begin with, MooPlayer doesn't require installation: you only need to copy the app's files to your hard drive. Also, being open-source, anyone can develop plug-ins of their own in order to improve the player or add new functionalities to it.
MooPlayer supports the most popular formats both in audio and video. As for the latter, it can also display subtitles and lets you use zoom to play a video in double or half its original size.
On the downside, I personally found the program's interface design to be quite annoying. The arrangement of button and other controls is fine, but having a cartoon cow constantly flashing on the background is not my cup of tea.
MooPlayer can play both audio and video, supports subtitles, doesn't require installation and is open-source. Pity the interface doesn't go along with these nice features.