Quick File Locker is a free tool which allows you to hide - and optionally encrypt - files and folders.
By default the program installs an Explorer shell extension, enabling files and folders to be locked from their right-click menu.
Alternatively, you're able to select files and folders from the main interface, and process them as required.
When this works, it really is quick and easy to use. Unfortunately, it doesn't always work.
For example, if you have the main interface open, then the right-click Lock option is blocked - it won't do anything until the main program is closed.
The right-click Lock doesn't work in some folder locations, either, complaining that "a required privilege is not held by the client". We can understand the basis for that - Windows user accounts don't automatically have the security rights to modify files in every folder - but it's still handled very poorly here.
Quick File Locker is easy to use, but also too buggy right now to recommend. Check again around version 1.2.