If you own a Mac, get Aperture. It's $79 (that's the full version). You can ONLY buy it via the Mac "App Store" (it's an electronic download.)
If you own a PC, you have a decision to make. Adobe Lightroom is $150. It works much like Aperture. Basically these are "image adjustment" and "digital asset management" apps. They're designed for photographers who have to take and manage lots of images but want to be able to quickly and easily adjust the images. Note I said 'adjust' and not 'edit'. It does do some editing, but it's fairly limited.
These DAM/adjustment apps are basically what you'll need 98% of the time.
Just occasionally you'll want to do something which requires more powerful editing... not just adjustment. For that, you'll want a true image editing app such as Photoshop.
The full version of Photoshop is very expensive (usually there's a way to qualify for a substantial discount, but it's even expensive when purchased at the discount price. For example... I got my first edition at 50% off but that was because I had a coupon that came with my graphics tablet. But even at 50% off I think it was still $300 (I think the official list price of Photoshop 6 is $699 and that's not the "exenteded" edition!)
But there is Photoshop Elements. Elements is $99. It contains the VAST majority of anything a "home" user would probably ever use and is fairly powerful. It's in the lineup because Adobe knows the average home user isn't going to spring $700 for a copy of the full version of Photoshop.
GIMP is free, but the drawback with GIMP is that it doesn't edit RAW. You're limited to 8-bit JPEG editing with your shots.
The advantage with either Aperture or Lightroom is that they are very powerful and yet still very easy to use. The advantage with Photoshop Elements it that it actually works like Photoshop and, for most people, it does everything they'd want. But if you do decide to upgrade to Photoshop (the full version) you'd already be familiar with how things work because you've been using Elements.
I used GIMP, Pixelmator, iPhoto, Photoshop Elements, Photoshop, and Aperture. ...of those, I LOVE Aperture. I can process whole batches of images in the time it takes most people to process just a few images (the workflow in Aperture is very powerful and fast. If you shot 10 images in the same conditions then you can actually apply the white balance, exposure, levels or curves adjustments, and/or highlights and shadows adjustments, etc. to ALL of the images at the same time. It's also VERY good at managing huge volumes of images... it can even manage offline images (e.g. it knows that I took images that are stored on external disks and it knows where the master image is located.) That means it's not necessary for all of my images to be able to fit on my hard drive. Lightroom is Adobe's product which does the same thing as Aperture... I can't really say I have experience with Lightroom. I used it back when it was in beta (before version 1.0). At that time, I liked Aperture better. Today, they're on version 4 and it's much more advanced. Lightroom is now more popular than Aperture.
