VLevel is a dynamic compressor which amplifies the quiet parts of music. It uses a look-ahead buffer, so the changes are gradual and the audio never clips. It is a command line filter and a LADSPA plugin, so it works with XMMS.
AFAIK there is no other plugin of its kind. This is invaluable in listening to high-dynamic-range music (like most classical music) in a noisy environment. It is a shame that portable audio players have not implemented this kind of algorithm as a feature because it would be great for headphone listening outdoors.