Spotlight is a great tool, especially with the updates that came in iOS4. As you probably know, iOS4 added folders. Before long, users had more Apps crammed into one page than they would have ever had in their entire iPhone before folders were introduced.
Unfortunately, this can mean it's hard to find an App sometimes. So why page through and check through individual folders when you can just hit the Home button from your Homescreen or page over to the left? This will bring up your spotlight search. Now type in the first few letters of the name of the App you are looking for. This is usually enough to narrow it down to find the App in the dropdown list.
This is also great for finding contacts quickly. Rather than going to your contact list, you can do the same thing. If your friend's name is John, hit up Spotlight and type in John, all the Johns in your contact list will pop up. It beats having to search and scroll around.