One of the best resources is the Seymour Duncan wiring diagrams page. Here you can find many different diagrams for numerous pickup combinations presented in clear diagrams.
Fender keep many of their wiring schemes online in their service diagram pages Telecaster Stratocaster
Another page with some interesting alternative diagrams is Phostenix' Guitar Wiring Diagram LibraryYou will also find links to other resources here.
Below are a few of our own diagrams.
Starting with perhaps the simplest wiring scheme of all, one pickup with volume and tone. This diagram is is a slight variation on others you may find because it has been wired "50s style".In a nutshell, the capacitor is wired so that highs are maintained when volume is rolled off. "Normal" or modern wiring can result in the tone muddying up as the volume is reduced. We call it Les Paul Jr wiring, 50s style.
One thing that is sometimes confusing is the layout of lever switch lugs. Some diagrams have them with right hand staggered, others with left. Switches come in both styles as well. The important thing to remember is that in the majority of cases the stagger is irrelevant; the lugs on each side correspond to the same position, no matter the stagger. So lug 1 right hand side on a left side stagger switch is exactly the same as lug 1 right hand side on a right side stagger diagram.
4 Way switching - We have produced our own wiring diagrams for both bridge/parallel/neck/series and our personal favourite bridge/parallel/series/neck.
We've included this Greasebucket diagram as it can be hard to see everything on the Fender diagram. Here's a great video of Mike Eldred from the Fender Custom Shop talking about the Greasebucket circuit on the La Cabronita single pickup guitar.