In order of cost and complexity;
Picasa 3, which is free, and does most of the basics of what people need, and is very easy to learn, and is none destructive.
Gimp, similar to Photoshop CS*, and is free. More complex than Picasa, and not as refined, but there is probably loads of videos to do a lot of what CS does, though not as sophisticated. It is a global, and pixel level editor, in that you changes to selected pixels through the use of selections.
Photoshop Elements. costs about £50, and does a lot of what the full CS* does, and has the benefit of being a pixel level editor, which Picasa is not, and also has a RAW editor. (a cut down version of Adobe Camera Raw included in the full CS*) Loads of tutorial vids on the internet.
Pixlr is a web based free editor, in that you upload your images to their website to edit, in a similar interface to Photoshop. Obviously you need to be connected to the internet to use it, but if you are on a computer without any editing software, but has an internet connection, then it's very handy.
LightRoom is primarily a RAW processor, though you can edit Jpegs almost as well. It can be had for about £90. LightRoom 4 has the same RAW processing engine as the Adobe Camera Raw plugin included in CS6. It can do other things.
And then there is the full Photoshop CS*, it can do probably anything you want to do to a Jpeg, or RAW file, and the vast majority of people don't get anywhere near using all of it's features. I don't, and I've taught classes about it in the past. You tend to learn what you need to know, and there are thousands of vids on the net showing you whatever you want to do. But it can be daunting with so many options to the newcomer.
It is expensive too, at about £600,

though can be had for about £190 if you're a student, have a child in school, or even if you are only on an Adult Learning/Evening course. Almost all Adobe products can be had cheaper this way.
IF I was starting from scratch, I would go for Elements, as it has the best balance of features, power, and easy interface to learn. Though Picasa, Gimp and Pixlr are all very good free options.
Not many people need CS*, but many want it.
There may be other options people may add.
