Hello everyone!
I recently purchased a camera that can save RAW images, and I am pleasantly surprised by the amount of OpenSource and powerfull software for RAW processing.

Darktable is one of my favorites. You can edit by light and color zones, create masks, use multiple instances on each filter, profiled noise reduction for some cameras, support LensFun, among many other things. It also has a great equalizer with good presets. I love it. Some people say it is the OpenSource Adobe Lightroom. Only available for Linux and Mac.
http://www.darktable.org

RawTherapee, I love this program too. Perhaps less options than Darktable for editing, but it seems more powerful and a little more professional in its features. It has a powerful noise reduction for high ISO, which I use a lot. Also the "Defringe" filter to reduce this kind of chromatic aberrations.
http://rawtherapee.com

UFRaw, I think it is one of the best known in the OpenSource world for being one of the first. It has a plugin for GIMP. It is powerful and easy to use. Sometimes I use it to reduce some chromatic aberrations.
http://ufraw.sourceforge.net

LightZone, I just discovered this program. It seems a very good program, has a very good user interface, but something it is heavy (I think it uses java). It has a lot of filters. Also some predefined styles that you can find out there.
http://lightzoneproject.org

Adobe DNG converter, It is not open source, but free. If your RAW format is not compatible with some of the above programs, you can convert your RAW to DNG with Adobe DNG converter. Also works with Wine.

Other OpenSource programs related to HDR, RAW and photos that I have not tried much:

Photivo :
http://photivo.org

Luminance HDR :
http://qtpfsgui.sourceforge.net

Raw Studio :
http://rawstudio.org

Delaboratory :
https://code.google.com/p/delaboratory

HDRMerge, It seems that this is something interesting:
https://github.com/jcelaya/hdrmerge
http://rawtherapee.com/forum/viewtopic.php?f=1&t=5426

DCRaw, used by many of the above programs.
Kipi -plugins , Hugin tools , lensfun , Digikam , Krita (Surely you can find them in the repository of your Linux ditribution).

So, lots of fun guaranteed

Recommended reading,
A Review of FLOSS Raw Processors
CiC: RAW file format
The Open Source Portrait (Postprocessing)

I would love to learn how to use raw files, but I would first have to acquire an extra hard drive to hold all the files as they are gigantic.

There is another one: gtkrawgallery

@Odinbc. Thanks for the links, very entertaining reading. I agree with the footnote on the first link. Only a few months ago I use these programs and I already saw improvements in the development with new features.

@jamac4k. Gracias!, I did not know that program.

@2-ton. There are many tutorials out there, that's good. Also you should be encouraged to experiment with these softwares. In most of them you can easily reverse changes in filters to see and test what each filter does.

For those using Ubuntu or family, the next is an excellent PPA with many of these updated software:
https://launchpad.net/~dhor/+archive/myway

The software I got with my latest camera - Olympus Stylus VH520 also supports Raw film editing.
I had the LightZone editor before I realised it did raw.

Good point sallyanne, is it Master your using?
The OEM RAW convertors are fine for starters when using their own file format, .crw .cr2 (Canon) or .nef .nrw (Nikon) or .orf (Olympus), etc. Of course with experience you want more.