Disk Drill is a free tool for finding and recovering anything from whole partitions to accidentally deleted files. It supports a wide range of file systems, including FAT, exFAT or NTFS, HFS+ and Linux EXT2/3/4.
The program requires installation, a problem as you might overwrite data you're trying to recover. But after that, it's all very straightforward: click "Recover" for your chosen drive, wait for the results, optionally preview any images you've found, then select whatever you need and recover the files with a click.
As with all undelete tools, the recovery process can take some time, but Disk Drill handles this better than most. If it's tying up to many file resources, you can pause the scan and resume it later. Or you can save the current scan session, close the program, then reopen it later and carry on where you left off.
If you don't have time to fully scan a drive right now, Disk Drill provides an option to create a backup image. That probably won't be practical for a full-sized hard drive - the file is going to be huge - but might be handy for external drives.
There's also a surprising extra in Disk Drill's "Recovery Vault". Tell the program to protect a drive and it saves copies of anything deleted from your chosen folders (Documents, say, or Pictures). If you accidentally wipe something then it can be recovered much more quickly from the Vault, and there's no chance of some vital file being accidentally overwritten.
It really needs a portable version, but otherwise Disk Drill is a great undelete tool with some valuable extras