Any Freeware to Undelete Items Emptied from Mac Trash?

I received a question from a student, asking me this Mac related recovery question:

I just emptied the Trash bin on my MacBook and later found I just made a big mistake as I have deleted an important folder (full of desperately needed documents & pics). From the net I know that as long as those files are not "secure deleted", chances of recovery are there. But I have spent a lot of time working on it by trying several Mac undelete software, none of free trial versions allow me actually recover my folder & files unless I register the application (which means I have to pay for it). Please help me out and give me some freeware for undeleting trashed mac files. PLEASE tell me I am still a student. I'm desperate for help.

[update on June 20, 2016: if your MacBook is with a solid state drive (SSD) instead of a hard disk drive, things can be quite different. Simply put, when you erase a file from SSD-based Mac, it’s beyond recovery. This is due to how the “Trim command” organizes a file in a SSD. Thanks to Lorena for pointing out this. Read her macbook tip here.]

Well, unless the Mac disk has been completely destroyed or broken, there is a chance to retrieve the files. So, my first advice would be try your best to protect your Mac machine from any physical damage.

And then the next step to recovering your data is to stop using the drive containing the lost files. Mac OS X overwrites free space very quickly with temp files. If your missing files are on your boot drive you will want to shut down your Mac right away.

Freeware to Undelete Trashed Files on Mac

If your Mac is running OS X Snow Leopard or above (including latest Yosemite, El Capitan), you can rely on this powerful backup tool - Time Machine, which is equipped with Mac machine that can backup almost anything and everything on a Mac timely. Once you deleted or lost some files, just enter your Time Machine browser, you'll easily restore your trashed Mac Items. This video shows how:

If you haven't set Time Machine or it just fails to retrieve your data, then I suppose the only way is to use a third-party Mac data recovery tool, see this list of totally free Mac data recovery software.

In fact, when a file is deleted off your Macintosh hard drive or partition, it's only the address of the file that is deleted from the files table. It's like having a huge book that has come unbound and lost its table of contents. The pages are there, but without the table of contents, it's much harder to find a specific chapter. Mac undelete software rebinds the book and scours the deepest parts of the hard drive that you can't access.