There are several more "focused" Google search engines. Each one in itself is pretty nifty. The U.S. Government link will let you search within government sites. So if you want to do a search about space, you'll come up with NASA web pages, for example, minus all the web sites put up by individuals or folks trying to sell you an opportunity to name a star after yourself.
If you don't want to try out all these new sites, there are plenty of tricks you can use to enhance your search at plain old Google. Start by clicking the "Advanced Search" button next to the quick search box. You can do all kinds of strange and wonderful things from the advanced search screen.
The first boxes let you control how Google looks for your search terms. The boxes are pretty self-explanatory. In the first box ("with all of the words"), you can put terms that absolutely must be in your results. In other words, if you're looking for web pages about The Color Purple, you don't want it to return results that only had "purple" in them. The most popular site could be about eggplants, in that case! Of course, for a search like that, you'd probably want to put "The Color Purple" in the box that says "with the exact phrase."
The box that says "with at least one of the words" is handy when you're not sure which word the authors might have used, and you don't want to miss any good sites. For example, you could put in "illness sickness syndrome disorder" and it would pull up sites that have even just one of those words.
Maybe you're looking for information about flea treatments for dogs. In the box "without the words" you could put "cats," if you want to avoid having to plow through info on felines.
You can put in limits on dates if you want to make certain a site has up-to-date information.
You can tell it to either leave out certain domains, or search ONLY in certain domains (like .com, .org, .edu., .gov, etc.) Maybe you're looking for some academic or scholarly information, so you might want to exclude .com sites, because they can be more commercial. Maybe you want to only search .edu sites, because these come from educational institutions and you could find out what academics are writing on the subject.
I'd tell you more, but it's making me tired just thinking of it all! Check it out and let me know what you think. I'm playing around with it all myself, and will probably cover each of these more in-depth later.