Advanced RAR Password Recovery http://www.elcomsoft.com/arpr.html
That's what I used in the past. If you have some idea of what password is, like if it's a word and not just random numbers and letters, and how long it is, you can get it done fairly quickly.
It cost $30USD, I'm not sure what the trial limits are.
They also have one for ZIP files, which for me I found worked much faster but it only does ZIPs.
If you got this via torrent, usually there is 2 ways to get the password.
1. If there is a web address in the file name; eg: www.joeblow.com then that is the password.
2. There is either a text that is not encrypted within the RAR file or the text file is just part of the set of files and not within that RAR itself.