Shooting in manual and trying to get your image right in camera will get you really far in photography, but often you want to go just that one step beyond a great photo to an amazing photo. Editing software or post processing programs can take your image that last little step. The big problem, there are a ton of choices, how do you know which one to get? And then worse how do you learn how to use it?
I am going to split up the programs I know about into price points. Some programs I have more experience with than others so I will be able to provide better feedback on them, that doesn't mean they are better than the other programs necessarily, though they may be.
Picnik - Picnik is a free online editing program. You can upload images from your computer to edit or edit images that you already have hosted online at places like flickr, facebook, photobucket etc. It has basic editing options under the edit menu such rotating, cropping, adjusting exposure and contrast, saturation and color temperature, sharpening and red-eye. Most edits, other than red-eye are applied to the whole image, you don't really have the option to apply it to just parts of images. There are also effects you can apply under the create menu and adjust like making images black and white, sepia, boost etc. If you stick to the basic edits and maintain a light hand, especially with things like sharpening you can give your images a decent little boost with picnik. Personally I would stay away from some of the more heavy handed effects.
Photoshop Express - Photoshop Express is another free online editing program. You have to create an account, which is free. Again you can upload from your computer or use images you have already posted to several sites. The editing options are similar to picnik a little harder to navigate for me since it seems like you have less control than I am used to. It shows you varying levels of the feature you have selected and you can either pick one of those or adjust the slider. Like picnnik, edits are applied to the whole image.
Gimp - GIMP is a bit different than some of the other free options, it is a downloaded program not online. A big plus for it (beyond being free) is that it can run on both windows and mac based computers. I have never used gimp but based on what I have read it allows you to do much of what photoshop does, and it allows you to work in layers like photoshop and similar programs do. Which is a great benefit when you want to edit just part of an image. It may even be able to edit RAW files but I haven't tried this myself.
Picasa - Picasa is a free image editor provided by google. It works on PCs and Macs. I haven't used it, the edits available look similar to Picnik and Photoshop Express.
Canon's Digital Photo Pro - If you shoot with a canon DSLR it probably came with a disk including a program called Digital Photo Professional. This program is a free RAW editor that you can use to process RAW files taken on a canon camera. This is the program that I used when I first started shooting in RAW and I wanted to test it out before investing in any other programs. I found it really easy to learn and it did a good job editing my images. You can do really subtle edits or more extensive ones, but you an really only apply them to the whole image. It actually does a really good job applying noise reduction to RAW files.
Conclusion: If I had no money to spend, and I shot Canon (which I do) I would start with DPP and if I needed more editing than that I would try to learn how to use GIMP, it seems like the most versatile of the free programs.
Paint Shop Pro - Paint shop pro is owned by Corel, it used to be owned by JASC. This was the program I used when I first started editing my photos. I found it easy to learn, and just about any tutorial for photoshop I could usually find a way to duplicate. The program works with images in layers similar to photoshop. You can apply edits to just part of the image using masks and you can record actions to speed up your processing if you find you use the same steps on a regular basis. This program had a couple of limitations for me, first it wouldn't run on my mac, second it really isn't as widely used as Adobe programs meaning there aren't as many free actions, tutorials etc which can feel very limiting. I believe you can use it to open RAW files, but I never made use of that feature. There is a free trial on the Corel website.
Photoshop Elements - Photoshop Elements is made by Adobe, the same company that makes Photoshop. To an extent it is a bit of a watered down version of PS. I have never actually used it. It edits in the same way, layers. There are mac and PC versions. It is a really common program and there are a lot of tutorials out there on how to use it. More free actions available compared to PSP but not as many as PS, you cannot create your own actions in PSE. I believe you can edit RAW files in PSE as well, again I have no personal experience with this. Adobe offers a free 30 day trial on their website if you want to give PSE a try.
Conlusion: If I only had $100 to spend I would probably go with PSE. While PSP was good to me when I used it, I think PSE is a bit more common which will make it easier to learn. Either way I would start with a free trial.
Both LR and Aperture are mainly RAW editors. They both also provide for photo organization, tagging, searching, printing etc and can be used to edit jpgs. Both have free 30 day trials available. LR works on both macs and PCs (and one purchase of the program gives you a disk that will install on both not just one), Aperture only works on macs (no surprise there since Apple makes the software). Based on the reading I have done the differences between this programs are pretty minimal. And the price point is very similar. I personally went with Lightroom after downloading the free trial and loving it. I think there are probably more resources, presets, tutorials etc for LR, and it intergrates with photoshop very nicely. Both programs pretty much edit the whole image, though LR has an adjustment brush feature that lets you apply an edit to just part of an image. Editing is very fast, I can edit one image in 10 seconds or less, a whole shoot of 200 shots in under 1 hour. Especially handy in LR is the ability to make your own presets, I have an import one that is applied to all my images on import, sometimes that is all the editing I need to do, and there are a lot of free presets available on the web to use. Also copy and paste of your edits and the sync/auto-sync features come in very handy when editing a large group of images.
Conclusion: If I had to recommend one I would have to go with Lightroom since it is the program I use and love.
Photoshop - This is pretty much it, the standard for photo editing. If you can afford it get it, though honestly I use LR for most of my editing. When I need to do things LR can't handle I take the image to PS to finish it up. Again a 30 day trial is available from the Adobe website. Photoshop lets you edit using layers, layer masks, adjustment layers. You can apply any edit to any single part of the image. It can also be used for some design work, I use it to create all the pages for my annual Blurb albums.
For all of these programs if you are a student or a teacher look into buying the software using an education discount, it will save you a lot of money especially on the more expensive programs.
In another post I will cover how I use the programs I have and how I have learned what I know about them.