There are plenty of photo editing software out there and if you have been in photography for any length of time you would have come across some tutorial asking to at least make a levels adjustment to make your photos stand out. [ A levels adjustment has an affect on the contrast of the image. Mostly it is done to make the image to have more of a punch. Our eyes inherently like sightly higher contrast images] Now you can do this with the image editing software which comes bundled with your camera. These do a good job for the most part but if you want to do high level of editing then you head towards the de-facto industry standard- Photoshop form Adobe. If you talk to photographers they would point you towards Lightroom. Both are amazing software's but they come at a price.
If you happen to be on the Linux side of the table the only way to run these is to try and run it with a windows emulator like WINE or you run a virtual machine machine running windows on it and run the software that ways. If you are on the Windows or Mac side and don't want to spend so much money then there are few alternatives which run on Linux, Mac and Windows.
It was the first photo-editing software I ever used. Though I could not make much sense of it when I tried it for the first time, but now I use it a lot. I have used Photoshop and when you compare it to GIMP there are times when GIMP falls short in, at least in some cases but as the saying goes "you get what you pay for" and with Photoshop you just get anywhere between little bit to a whole lot extra. Having said that you get a lot with GIMP for free. In fact I know a lot of photographers who use Lightroom along with GIMP and don't feel the need to purchase Photoshop.
There are many variants of GIMP like GIMPphoto or GIMPshop and if you are in new to photography then I would recommend you start of with GIMP before you drop money to buy Photoshop. Both Photoshop and Gimp are destructive forms of editing. In other words if you edit a photo and save it the changes are permanent. [Unless you keep a copy of the original photo.] This is not the case with Lightroom. Every change you make is undo-able. The original raw file is available for re-editing as many times as you like and all the changes made can be undone. [Unless you replace the original jpeg]
Lightroom works natively with camera RAW files unlike GIMP and Photoshop which need plugin's to work with RAW. This apart from many other reasons this software is a huge hit with photographers all over the world.
Rawtherapee - Alternate to Lightroom
Lightroom unlike Photoshop is also a workflow tool apart from being a non destructive form of editor. Now if are looking for a cross platform RAW editor currently I could find only Rawtherapee. There are many other RAW editors which run currently only on Linux or in some cases on Mac's as well. [Not on windows] Rawtherapee has many of the features which Lightroom has but it is not as intuitive to use. The interface takes some time to get use to. However, once you get a hang of it you can do a lot of things with this software.
Another software which I tested and liked a lot was Darktable. It was quite intuitive to use and currently runs only on Linux and Mac's. It has most of the features which are there on Lightroom and editing on it is a lot of fun. If you are in the world of Linux then I would recommend this software highly. Now if you are a windows user then I would recommend you install a virtual machine and try this software. If you have never installed a virtual machine it is worth the hassle to learn how to install a virtual machine and then install this software. If you are on dual boot then do give this software a try.
If you are on a budget, but would like to start editing your photos and try out a lot of the Photoshop or Lighroom tutorials for free this is a good place to start. Happy Editing