According to the official Google Docs Blog, here are the dozen new file types you can view in the Google Docs Viewer, including two different kinds of Microsoft Excel files, and .PPTX file compatibility for PowerPoint 2007 and 2010:
Microsoft Excel (.XLS and .XLSX)
Microsoft PowerPoint 2007 / 2010 (.PPTX -- before, only .ppt and .pps were supported)
Apple Pages (.PAGES)
Adobe Illustrator (.AI)
Adobe Photoshop (.PSD)
Autodesk AutoCad (.DXF)
Scalable Vector Graphics (.SVG)
PostScript (.EPS, .PS)
TrueType (.TTF)
XML Paper Specification (.XPS)
Keep in mind that you can't create documents in these formats using Google Docs -- you'll still need the original applications to do that.
I gave this a try with a Photoshop document, and it was an unusual feeling to see that "view" link in Gmail when I sent myself a 1.8MB .PSD file. And, when I clicked on that link and the Google Docs Viewer appeared, the Photoshop file rendered just as well as it does inside Photoshop itself, perfect for a quick peek.