Multiple document windows are a little known Photoshop feature that saves a lot of zoom effort! To open a second window for an open document, go to Window > Arrange > New Window for [filename].

This comes in very useful when you need to zoom in to a high level but would still be able to see realtime changes to your document at 100%. I mainly use this when designing icons, with two or three copies of a document open simultaneously, each at a different zoom level.