I didn't have an editor on my Macbook (other than the iPhoto, I mean), but after reading the comments here, I installed GIMP and I really like it.
I was going to suggest Pixelmator as well. I've been hearing a lot of good stuff lately, and the current version has many of the important tools from Photoshop, like the healing brush, that most alternative editors lack. If Adobe insists on jacking up prices AND upgrading every version, I may have to give it a try.
To edit all my photos (cosplay, family, scanned art, and dolls) I use a combination of PICASA (free) and GIMP (free!). Picasa has a really wonderful "I'm feeling lucky" edit option, and it's actually pretty darn good most of the time, and it has some good effects and a good retouch option, along with other editing options as well. GIMP is fabulous because I can both draw on it and edit it. Whenever I edit on GIMP, it means I'm doing a little more hardcore edits, playing with colors, adding or subtracting stuff, erasing stuff, smoothing out colors (and skin! lots of skin!). If anything, if you don't wanna spend any money at all, those are two GREAT options for a combo.
As a photography student I can say one thing. Lightroom. Adobe Photoshop Lightroom is the best photo editing software there is (I think). Or iPhoto if you're a mac fan, but it's not quite as good.
I'm quite new to photoshop myself, so I only use the basic stuff on ps. But I recently found something that's pretty fun to work with and can have a pretty good result too. It's great for someone with limited editing skill or someone who don't have ps. The photos below are edited by apple photo apps, all but one I use are free. (But the texts on the photo on the right are made by photoshop.) The one I paid $2-$3 for have some edit program similar to photoshop and the free one usually just come with preset effect. It's not necessary to use the iphone/ipod camera. You can just take the picture, send/transfer it to your apple device, edit it there, then sent back to your computer afterward. (Sorry for the same photo spams >.< I had too much fun with the apps )
Lightroom isn't worth it unless you're shooting raw. The tools for editing RAW files are really excellent, but if your camera doesn't do RAW output or you choose not to use it you'd be better off with Photoshop or another editor (GIMP, Pixelmator, etc) because it can do basically all the same things Lightroom can with a JPEG, plus a lot of other pixel-level editing that Lightroom isn't terribly good at. Want to add text to a photo? Lightroom cannot help you. Want to do speech bubbles in your photostory? Lightroom is not good for that. Want to adjust the exposure and color balance of a photo in RAW format? Lightroom is an excellent choice. Also, comparing iPhoto to Lightroom is really not apt or fair; they're nowhere near equivalent. iPhoto is a consumer photo editor. It's made for people who take photos casually to edit and organize their photos. Lightroom is a photo workflow solution for professionals or serious hobbyists, and the Apple equivalent would be Aperture. iPhoto is a lot more like Picasa, IMO. Basically if you're not 1. pretty serious about photography, and 2. shooting RAW, the $150 you'd spend on Lightroom would be far better spent elsewhere. It's important, if you're going to recommend software, to think of who you're recommending it to and what they're planning to do with it. I wouldn't recommend LaTeK to someone who wanted to write a shopping list, and I wouldn't recommend Lightroom to a casual hobby photographer who just wants to crop photos, adjust contrast, and add the occasional text.