app to hide/lock folders

Versiontracker is your friend.

This was a search for "hide folder". You could also try "lock folder" or "encrypt folder".

If you have something you truly want to keep confidential, you might want to encrypt it. I believe a very savvy Mac user could find a hidden file if he wanted to.

Also, File Vault is built into Tiger...but it does your entire home folder. And if you share only one account with other people, you will be locking them out of the home folder.

I haven't tried it, but maybe Locker is what you're after.

Can't Disk Utility do the job?

Just did a search you could create a password protected disk image

http://docs.info.apple.com/article.html?artnum=107333

Not sure if that's what you really wanted.