Before you panic, power down your computer. Don't just shut down, power it down. If there is no power button, unplug it. Every additional access to your computer will potentially result in data loss, unless you don't boot the hard drive, or access it from a normal file system. There are at least 6 data recovery tools that allow you to recover data without directly accessing the file system:
Each provide a bootable CD that will boot most current Macs. Contact the vendor in question to ensure you have one that works with the system, and when you bought it. Attaching an external hard drive at least as large as your internal hard drive will allow you to recover data while booted off their recovery tool. Booting the CD while sometimes possible with the 'C' key is not always possible. In that event use the startup manager.
Note, if the system was bought new on or after 10.7's release and has had Lion installed on it, or just has had 10.7 installed on it, once your data is safe, you can attempt a recovery of Lion itself with the Lion Recovery partition, if you don't erase or replace that hard drive first.
Select the CD from either interface, and then select either the return key in the former, or click on the right pointing arrow at the bottom on the latter.
If you can't get the CD to boot off your Mac, you may want to zap the PRAM if the machine is under 4 years old, or check if the PRAM battery needs replacing first if it is older.
Regardless, unless you can get the machine to boot all the way to the desktop, to backup files, you probably shouldn't run Alsoft Disk Warrior, or Micromat Techtool Pro. You also shouldn't attempt a software installation, unless your data is backed up, as this will only decrease your chances to recover your data later. Disk Warrior can be used to repair the directory, but should only be done once you know your data is safely in two other locations.
Some Apple authorized dealers offer non-destructive data recovery services too which may have more powerful tools.
http://www.drivesavers.com/ has become an expensive, though very well recognized company for data recovery when the above tools don't work. No utility is perfect, and hence the reason you should always keep at least two backups of your data at all times. Still it is good to know in a pinch what your options may be.
And remember, backing up (tip on backing up link) your computer is not an option. It is required for the safety of your data. Backing up means having at least separate two copies off the computer of your data. They can be printed, burnt to CD, DVD, stored on an external flash drive, or an external hard drive. A simple action of deleting a file by hand can happen quite by accident, but also there is no such thing as permanent media out there. All media eventually fail. A remote backup will also ensure you won't be at as great a mercy of loss or theft. And monitor the consistency of your backup to ensure you don't have to remake it. Only you know your tolerance level for how much time you have to recreate data from scratch. Use that to determine how frequently you should backup, and just do it more frequently.