From the ALSA wiki
This is a list of ALSA plugins. Plugins are used to create virtual devices that can be used like normal hardware devices but cause extra processing of the sound stream to take place. They are used while configuring ALSA in the .asoundrc file. If you know anything about one of these plugins, please add it to its page. You may have to create the page first. In that case, don't forget to append
[[Category:ALSA plugins]] to the text in order add it to this category. If there's even a plugin missing, please add it to the list!