If you listen to pop and rock music, you may be interested in the lyrics of the songs, in which case the Lyrics plugin will give you a lot of help, bringing the lyrics of a song in a separate pane of Winamp in real time. (Try this for Foobar).
When you have mp3s that were recorded at different levels, or if you are listening at night and you want to enjoy the quiet parts of the songs, without having to decrease the volume when loud parts come up, you can use the KMG DSP Dynamic Volume Winamp plugin.
Check this post on how you can save your Winamp equalizer presets (if you use the equalizer).
If you listen to music when you go to sleep and you want your computer to shutdown after Winamp finishes its playlist, you can use the Winamp Shutdown plugin. AutoShutdown is a standard feature in AIMP.
Get the One Click Show & Hide plugin, if you want sometimes Winamp to remain minimized to the system tray, disappearing from the Desktop, the taskbar, even the alt+tab chain. The plugin will let you do that using your mouse or a global hotkey.
If you are going to download a lot of music, you can use a utility such as ExtractNow that will monitor one or more folders for new archives and extract them automatically.
Check also this list of top free audio utilities anyone may need.