TestDisk is a free data recovery tool. Designed to help recover lost HDD partitions, TestDisk can also make non-booting disks bootable again and helps to restore your partition table[?], that was broken by faulty software, viruses or just human error (such as accidentally deleting a partition table).
Mac OS X
TestDisk is freeware data recovery software developed by Christophe Grenier.
If you had lost a partition someway, than TestDisk is just the thing you need. Because, before running some file-recovery software, you can try to just "enable" your lost partition and you will get everything back just as it was. Losing partitions can happen due to many things: software failure, corrupted partition table, accidental deletetion by a user - in all these cases it will be enough to repair the partition table to get your data back. TestDisk does exactly that.
But, it's up to you to select what partition you are looking for - TestDisk will display all possible solutions, some of them may even had been overwritten long time ago. So you need to choose the one you are really looking for.
The most recent version of TestDisk not only can recover lost partitions, but it can also do many other data recovery operations like restore MBR, rebuild a corrupt boot sector[?][?], recover MFT from MFT mirror, and even undelete individual files and directories.
TestDisk supports many file-systems, and can run under several operating systems - did I mention that it's Open Source?
But, there is always a fly in the ointment - the interface. Not going to say that it's bad - no it isn't. It's just: Open Source, you know - a black console window with white text on it. Not too much attractive, but at least it's not command-line. Some kind of a wizard will guide you through the entire process, describing every detail, so I think there will be no barrier for a novice to use it. Anyways, if you don't want to understand anything in data recovery, then TestDisk can create a log-file for you, so you can show it to data recovery specialist.
The official wiki-style website has some detailed step-by-step tutorials and pretty much info about how TestDisk works, how it searches for partitions and how it restores them. A curious person can learn every aspect of the recovery and perform it all by oneself - there is enough information for that.
TestDisk is indeed a useful tool and the first aid rescue if you lost a partition. Just try it, and in most cases - it will solve your problems. But if it won't - you shall need to try running some file recovery software, which will scan your drive for lost files, and try to restore them. By the way, TestDisk comes with such an application named PhotoRec, which just does the thing. Of course, commercial data recovery software may give you a better result, but why not to try freeware first?