you can use some publicly available MD5 password crackers like tmto.org, md5decryption.com, collision.net etc. Some other services are also available but they are very poor. Even these crackers don't work perfectly.
If it's a good password, as mmerlinn said it could take centuries. MD5 isn't encryption, which can be reversed, it's hashing, which can't be. The best you can get (and only if you live long enough) is one of the (possibly) many passwords that will produce the same MD5 hash (which will work in logging in - but only if you're still alive by the time the computer finds the first one). If it's a trivial password, like 5 lower case characters, it might get broken in a shorter time. (Which is why you should use a 20 character password consisting of random upper and lower case characters, digits and punctuation.)