For simple modifications also ToyViewer is nice, it's freeware. http://www.versiontracker.com/dyn/moreinfo/macosx/4580
Also iPhoto allows some modification, and there are plenty of other softwares too - have a look on Versiontracker and search for keywords, you can see what apps exist, if they are freeware or shareware, and how people like them.
AppleWorks can also open most image files, although it's not immediately obvious. You need to go through the Open dialog and use the popup menu to set the file format to "all available" before it will let you choose JPEGs. Also, set the "Document type" popup menu to "Painting".
Don't ask me why it doesn't allow for simple drag-and-drop like everything else...