boy with violin
Violin Mutes
Violin mutes are used for just that, keeping the violin quiet.  They also affect the tone of the violin, giving it a quality that many folks like.  If you see the term "con sordino" in a piece of music, it means that the composer wants the violinist to mute his violin.

Different mutes for different purposes.  The first type is a practice mute.  This is simply to keep the violin quiet during practice.  You can still bow as you would during a performance, but the violin will remain quiet.  The second purpose is as an orchestral mute.  This is for times where the composer specifically states that he wants the instrument muted during performance.  The mute keeps the violin much more quiet and alters the tone somewhat. An orchestral mute will help keep the instrument quiet during practice, but a practice mute is totally unacceptable in a performance setting.

Here are a few mutes that I can recommend. 
Rubber Practice Mute
This rubber practice mute provides high level of muting while preserving more tone quality than the metal variety.  Its purpose is to dramatically cut the sound level so that you can practice in an environment where quiet is necessary, yet you can still bow your instrument the way you would during a performance.  This is an outstanding product!
Tourte Double Hole Violin Mute
This mute produces a sound that is obviously muted, but not too muted, still produces a good sound on solo, and orchestral levels.  Very popular.
Tourte (Original) Single Hole Violin Viola Mute
Genuine Tourte violin/viola mute. Violin shaped, single hole. Substantial hard rubber variety with excellent orchestral or solo muting qualities.  (I prefer the two hole version.)
Mouse-Tro Orchestral Violin Viola Mute
This adorable orchestral mute captured as a mouse dressed in a tuxedo. Hand-painted.  They do a very good job muting the violin and are a great motivator for kids . . . and adults too for that matter.I've seen feedback that these have been coming out a bit stiff when they are new, but loosen up as they are used. 
Glaesel GL-3800 Wire Violin Mute
Sturdy wire frame easily fits over bridge while clear rubber tube mutes the strings. Brass inserts permit rolling and stabilize tube.
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and Accessories
Tourte Double Hole Violin Mute (Magnetic)
The difference between this mute and the Tourte Double Hole Mute listed above is that this contains a magnetic insert.  A second piece attaches to the violin's tailpiece.  When the mute is not in use remove it and attach it to the magnetic piece on the tailpiece.  This eliminates the mute's hanging from the strings.  (While I know folks who use this, I've never experienced a sound problem with the regular Tourte mutes.)
hush emoticon for violin mutes