QUOTE(spark @ Dec 14 2008, 11:11 AM)

How much do they reduce the sound by?
A tourte mute - the little black round rubber type that's usually used by most orchestral violinists (and soloists) - are probably the easisest and most convenient mute available.
QUOTE
Can you still hear enough to correct intonation?
Yes for most mutes. However, using heavy-weight practice mutes do mean you have to listen a lot more carefully.
QUOTE
If I'm going to get one, which one should I get?
A tourte mute would be the best place to start if you've not used a mute before.
Don't practice with a mute unless the music calls for its use, or you are having to practice somewhere where it's prudent not to annoy neighbours/other people sharing the same building etc. If you do, then you will find your tone production will suffer in the long run.
A note of warning though - not just to string players, but all musicians. When it comes to extensive periods of time practicing in rooms with little or no acoustic properties, a pair of musicians earplugs (which reduce the decibel level without reducing the range of harmonics your ear hears) are essential to preserve your hearing in the long term. Prices start at about £14 for a pair you can buy in a music shop, which is not an awful lot to pay to save your ears from permanent damage.