Paint.Net plugin lets you view and edit Photoshop PSD files
Download the zip archive, dump the included PhotoShop.dll file into your Paint.Net FileTypes folder (usually c:\program files\Paint.Net\FileTypes), and you're ready to rock. Pretty well anything in the PSD for which Paint.Net has an implementation will load just fine. Saving is another story, so you'd best stick to Paint.Net's native .PDN file when you're done working.
If you don't have Photoshop, the plugin is a handy for looking at PSDs your friends might send you to look at. It's also a nice way to transition yourself to a free alternative if you no longer require Photoshop.
Thanks to FreePSD.com for the template I experimented with -- it's been so long since I used Photoshop on one of my systems that I can't even find any of my old PSDs...