A friend and I were surprised to discover that Safari knows how to display Photoshop .PSD files! It opens both local and remotely served .PSD files as easily as a .JPG or other supported image file formats. In retrospect, this ability may be due to a shared system-wide library in OSX rather than Safari itself, as the Finder image preview (in column mode) and Preview.app also have this ability (similar to how Safari will also open and display .rtf files).
Related hints:
Print Photoshop files through Preview
Use Preview on Photoshop files
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030712051611750