Windows Media Player

• foobar2000 - While only audio, supports a wide range of formats without plugins, and has low memory usage and is extremely customizable.
• iTunes - Media player from Apple. Provides access to the iTunes Music Store (although not recommended as it bundles DRM with songs).
• Songbird - In alpha stage, integrates well with music websites. Supports syncing to other devices.
• VLC Media Player - Does not have a library feature, but plays almost any media format.
• Miro - Video only, features RSS downloading, BitTorrent, and support for video websites such as YouTube.