ImageWell and LiveQuartz
ImageWell is a quick image retouching/resizing tool that has a great workflow optimized for bloggers. You simply drag an image onto its canvas (for example, from iPhoto) and you can quickly resize it, add some effects (like callouts, etc.) and FTP it to your server. It can even copy an HTML Image tag with the correct URL for your image to the clipboard to help you quickly insert the image into your blog. The one thing it doesn't do is give you a Photoshop-like multi-layer environment for compositing multiple images. This is where LiveQuartz comes in.
LiveQuartz is a free image composition application that will be instantly familiar for Photoshop users. It has layers and allows you to create composite images.
Of course, neither is an alternative for Photoshop but until the universal binary for Photoshop comes out, they're going to save me from having to copy my images over to my Windows machine under Parallels to edit with Photoshop.