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There are a few out there. Check Versiontracker.com with the search word "encrypt". Most are free.

I use iCrypt. It's simple and works OK. I use it to encrypt some medical files and other personal docs. I could never get a handle and manipulating the permissions access. You could use FileVault on your system preferances but I never trusted it because I heard it had bugs and you could lose everything.
 
iCript (with its cool icon) is better for encryption of single file AND folder while Disk Utility creates password-protected .dmg folder ONLY.
 
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