You could try GIMP. It's freely available from gimp.org, and it's capable of doing all those things. I like it because the GUI makes more sense to me than programs like Paint.NET. It also has some pretty snazzy filters that are fun to play around with to get different effects. You can open PSDs (Photoshop Documents) and a variety of other filetypes, which is handy if you start working on a photo manipulation on a computer with Photoshop (for example, a friend's computer), then switch to a computer without Photoshop.
However, some people find Paint.NET a lot more intuitive. Although, being someone who tends to draw with a graphics tablet, I find the lack of pressure sensitivity and PSD support disappointing. I also find the colour picker a bit confusing - although a few of my Paint.NET-using . It should be fine for just editing photos quickly, though.
If you don't want to download any software, you could try Aviary's in-browser editors. I think Phoenix is the image manipulator, and from the times I've used it, it's pretty cool. I haven't done any serious editing with it, but it's good enough for basic stuff. All of Aviary's editors are available at aviary.com
EDIT: GIMP, Paint.NET and Aviary are all completely free.