Is there a Mac equivalent to Windows 7's library feature? Having several related files in different folders, I was looking for a way to combine them in the same view,  like the "Libraries" of Windows Explorer in Win7.

While Finder has great smart folder capabilities, I don't see any way to do this? Third-party tools are welcome.
Easiest way I can think of to do it is if you bring up the file info window by pressing ⌘ + i keys, and then add something into the Spotlight Comments field that you keep the same through the files you want. Rinse and repeat for the files. Create a new Smart Folder, for the search option go to Other... and it'll bring up the full list, which will include Spotlight Comments.