When Apple made the move from Tiger to Leopard, they decided to throw in more than a few features. One of those new features was the ability to search Wikipedia right from Dictionary.
To start searching Wikipedia, just open Dictionary (located in /Applications). Next, click on the Wikipedia button and enter a search term. Leopard will then browse Wikipedia for the answer.
You can access the dictionary from most applications by highlighting a word, right-clicking and selecting "Look Up in Dictionary."
Want more tips like this? Visit TUAW's Mac 101 section.