Mail User Agent
, that is, an E-mail program
A hideous piece of code with which you can spam people with forwarded news articles and old jokes... or you can use it as a wonderful tool for Internet collaboration.
Popular examples include (but are definitely not limited to) Mutt, Pine, Elm, Gnus, Eudora and Outlook Express.
Many MUAs also act as newsreaders. (Also, some newsreaders act as MUAs...)
See also: MTA