Smart Objects can save you a great deal of time if you are designing for the web in Photoshop. Furthermore they allow you to manipulate images without destroying the original.
Until recently I'd been using desaturate to convert photographs to black and white in Photoshop. In fact there is a much better non-destructive method.
Designing with grids is commonplace in design but the danger is that everything looks a bit ordered. Using grunge effects you can keep your grid and mess up your hair to look a bit cooler.
One day you might just be insane enough to want to write concentric text in Photoshop. It is quite illegible and likely to give you a headache but if you still want to do it read on.
A less well known feature of Photoshop is that you can create custom vector shapes and use them throughout your designs. Here's an of examples of how to do it.