let's pretend you need a serial number for windows xp pro.
in the search bar type in just like this - "Windows XP Professional" 94FBR
the key is the 94FBR code.. it was included with many MS Office registration codes so this will help you dramatically reduce the amount of 'fake' porn sites that trick you.
or if you want to find the serial for winzip 8.1 - "Winzip 8.1" 94FBR
just try it out, it's very quick and it works nicely.
here is another trick that works fairly decent for finding mp3's on the web (which is hard to do normally, to say the least)
say you want to get, for example, a Garth Brooks song. type this in the search bar - "index of/" "garth brooks" .mp3 the ones you want to check out first are the ones that say "Index of/" in the title of the search result. this technique allows you to easily pull up web folders with direct downloads. it will look the same as if you were logging into a ftp url.. i'm sure you can be pretty flexible on how you type that in, so long as you include "index of/"
i'm sure you can use this for more than just mp3's (it's not perfect but it has worked for me on a few occasions)
always make sure to use the quotations where i placed them. they help pinpoint the correct search results more accurately. just try it out, also if you want to learn how to do more with google look up "google hacks"