1. Large Icons (Snow Leopard)
One of my favorite features in Mac OS X 10.6 Snow Leopard is the Large Icon option. When you open any Finder window and switch to Icon view you can now drag the slider in the lower right corner to enlarge the view. In many cases this allows you to see the images in detail or even read the text of a document to find the one that you're looking for.
2. Preview inside of an icon (Snow Leopard)
This is kind of an add on tip to #1. Once you're in Icon View and you have the nice large thumbnails, if you click on a supported document you can actually preview it right in its icon without having to open it. Step through a PDF, flip the slides of a Keynote presentation or play a movie. Cool!
3. Change Audio Options (Snow Leopard)
The menu bar can have lots of icons representing System Settings for quick access instead of always having to go to the System Preferences. Now if you hold down the Option key before clicking things like the Sound Icon (AirPort Icon, etc.), you'll see several more options. For sound in particular you can set your input and output settings.
4. Change the Desktop Background directly from the Desktop
When I want to change the Desktop Background the fastest and easiest way to do it is to just Right Click on it in the Finder and choose Change Desktop Background from the Contextual Menu. This will take you right to the spot you need to be in in System Preferences.
5. Use any Finder Window to Launch Apps/Documents
Every Finder Window has a tool bar at the top. However, few users realize that you can customize it. You can even put your own Apps/Documents on it for quick access by simply dragging the icon you want right up to it. This way you'll be able to quickly access that App/Doc no matter which Finder window you have open. This is also handy when you want to open a document in a particular app. Just drag the document right to that Icon to open it in that app. When you want to remove the item just hold down the Command key and drag it off the bar.
6. Put Frequently Used Documents in the Sidebar
Although you can certainly put documents in the Dock for quick access, I also like to put a couple of my most frequently used documents right in the side bar of a Finder Window. There's one advantage to having them there. When you're in an App and you choose Open from the File menu, the Sidebar is listed and of course all of the documents in it are too. This way you can always open that document from within any App you want without having to go digging for it.
7. Use Labels
I use Labels throughout Mac OS X where ever I can. I really helps when you have a large folder full of items and you're looking for something. If it has a color label it will stand out. I also do this in Notes in Mail.
8. Quick Look instead of open
Quick Look is one of my favorite features. Just select a document and hit the spacebar to preview that document in a nice large window. However, what many folks don't realize is that this also works (at least in Snow Leopard) in an Open dialog box. Very handy when I'm trying to figure out which photo I want to open and they are just generic file names. I can sort the folder by Date Modified, but still see what they look like by hitting the spacebar. Once I find the one I want to open, I just hit the spacebar again to get rid of Quick Look and then return to open it.
9. Open it with a different App with Open with
I don't always want to open a document with the default app. For example, I may want to open a particular photo in Photoshop Elements instead of Photoshop CS4. So I can right click on the image and choose Open with from the contextual menu. It will usually list all of the apps that you have installed that can open that kind of document.
10. Show Path Bar
It's easy to find something with Spotlight, but sometimes you're looking a file and you don't really know where it is on your hard drive. There are a few ways to find out, but one way to constantly know is to choose Show Path Bar under the View Menu. This will show you a constant view of the path you're in and the file you have selected.