This is one of my all-time favorites. In nearly all applications that deal with text (web browsers, text editors, information managers, Pages, Mail, ...) you can select a portion of text and press ⌘E. This puts the selected text onto the system-wide find pasteboard which does not interfere with the normal pasteboard you use for cut/copy/paste. Now you can use ⌘G to search the next occurrence of your search phrase starting from the cursor location. Search backwards by pressing ⇧⌘G. Open the search panel with ⌘F and the search field is already pre-filled with the phrase.

By the way: I think they didn't choose ⌘G incidentally. On the keyboard G is directly above the key V. This way you can search and replace repeatedly by sliding your index finger up and down a little.

One more thing: The words system-wide above are important: You can put something onto the find pasteboard (⌘E) in your web browser, then switch to your text editor and search that phrase there (⌘G).

I just wonder why Spotlight doesn't use the global find pasteboard. I wonder every time I use it.