One of the fun things about learning to fly is figuring out where you're going to fly. The official old-school way to plot a course is to grab a sectional chart and unfold it on a big table (or in a small cockpit), then bust out the compass, protractor, and slide rule. It's fun to do things that way, but fortunately in the modern computer age there's easier ways. (I like AOPA's flight planner).

Google Earth is a great tool for answering the question of where you want to go. It's particularly good at showing terrain, since the primary view is overhead photos overlaid on an eleveation model. But Google Earth is also uniquely good at layering in extra information that helps figure out where someplace interesting to fly would be: what airports have nearby restaurants, hotels, parks, etc.

The default Google Earth layers are pretty good, but there are a few special layers from third party sources that are specifically useful for general aviation pilots. Here's a few I've found. Note: these datasets are often out of date and not suitable for navigation.

Airport placemarks (Sampas KML, JustSoar KML)

Google Earth's airports layer only shows big commerical airports unless you zoom way in. So it's handy to have a KML file with a zillion pushpins that are always visible. I prefer Larry Sampas' version because the display titles are short 4 letter codes. The JustSoar versions include runway geometry and are handily categorized as public vs. private.

Circle generator (KML generating form)

Very simple tool: draw a circle of a specific radius at a specific point. I have 100nm and 200nm circles drawn around my home airport.

Sectional Charts (GELib KML, ChartGeek)

These sources do a massive overlay of your familiar paper chart right on top of the Google Earth display. You'll want to adjust the layer transparency to see through it. Both versions also have a 3d polygon view of controlled airspace that helps you make sense of the smashed wedding cake of San Francisco Bravo. The Google Earth Library data is free, but based on a 2006-era dataset. ChartGeek is selling frequently updated datasets with lots of nifty features. They have a free San Francisco sample you can try.

OpenStreetMap (KMZ)

Street maps. Shows where people live: the terrain view of Google Earth isn't very helpful when trying to estimate how big a town is. I'm not wild about this Google Earth / OSM integration, but it works OK.

There's some other work out there integrating more aviation data into Google Earth: IFR routes, more geometries of the stuff on charts, etc. The core data source for all of these things is the National Flight Database, the government's publication of aviation navigation data. That data won't work directly in Google Earth, but its availability means geohackers can build their own products. Sadly the data is no longer free, it's $5 - $x00 depending on how much data you want and whether you need a recurring subscription.