Vista had its own version of a preview pane in Windows Explorer, which didn't work that well and didn't work for all that many file types, but Windows 7 gets it right.
Win 7's version activates via a button on the top right of Windows Explorer. It pops out and displays files like music, videos, HTML docs, photos, WordPad docs (but not Word's .docx flies) directly in the pane without having to open a separate app. It's not quite as good as OS X Leopard's Quick Look, which does support PDF and Word and Office docs, but it's definitely better than Vista's preview.
View our other Windows 7 tips and our continuing coverage here.