I'm huge on Google's Picasa:

http://picasa.google.com/

Here is a walkthrough & review of the software:
http://lifehacker.com/5059302/picasa-3-betas-new-features

It does all the basic edits plus a bunch more. Best of all is the way that it organizes and shares your photos. It's really great free software.

There's even a mac & PC version so you can use both without having to learn new commands or a new U.I. Personally, I keep all of my photos on a network attached hard drive and use the same library on both my Mac and PC. It works wicked awesome.

Skitch has a few more screenshot type copy-and-sketch functions. If you're after those types of things then the free Paint.net is for you: http://www.paint.net/

For my pro photography work, I use Adobe's Lightroom & Photoshop. However the price tag of those is a little prohibitive. Plus, I like the easy-to-use interface of Picasa for my family & personal photos.
Permalink | Report

No question, it's the GNU Image Manipulation Program!
Source(s):
http://www.gimp.org/

Permalink | Report

It's really a matter of opinion, but I've seen a lot of people use Gimp or Open Canvas Free. Both those programs should suffice for basic editing and any other needs.
Source(s):
http://www.gimp.org/
http://wistinga.online.fr/opencanvas/

Experience

Permalink | Report

For basic image manipulation you don't even need to download a program. Phoenix, a product of Aviary.com, is a online flash-based photo editor meant to emulate Photoshop. You can access and edit your photos from any computer, unless if you have a slow connection. It can do anything I've attempted.This may not fit your needs, but I thought I'd suggest something outside the box.
Source(s):
http://aviary.com/tools/phoenix

Permalink | Report

There's also https://www.photoshop.com/ which isn't half bad, depending on what and how much you want to do...and it has the added benefit of being web-based with 2GB of online gallery space.
Source(s):
Memory. Experience. :)

Permalink | Report

For its quickness and easy of use, I really like Picnik. It's a web-based photo editor. You can do all the basic things (crop, resize, add text, contrast, touch-up, red eye, etc) right from your browser at anytime, without signing up. I also really like the fact that I can pull (or save) photos from sites like flickr or facebook so easily.

http://www.picnik.com/
Permalink | Report

GIMP if you need raw power and lots and loads of complexity. Yeah, you've seen them say that, above.

But lots of folks don't need something that hefty. Maybe look at Paint.net, for one. Basic editing. Easy to use.

Check out picasa from Google. Good for photos. Superb slide show. Some surprising effects you can apply to photos.
Permalink | Report

Definetley irfanview. You don't want GIMP because it's really complicated.
Irfanview is totally free, and it replicates many of the functions of photoshop, without making them totally confusing like the GIMP does.
Resizing, adding filters, working with layers, all easy. Plus you can save as and open photoshop files (or just about any other format).
Source(s):
http://www.irfanview.com/

Permalink | Report