Google Maps and Yahoo Maps, side-by-side
I have created a demo showing Google Maps and Yahoo Maps side-by-side (both the AJAX and Flash versions of Yahoo Maps). The maps are linked together, so if you pan/zoom on one the others will pan/zoom at the same time. It's an easy way to compare them by their looks and performance. And if you look at the HTML you can glean some valuable tips on working around each one's quirks.
I got the idea of a side-by-side comparison from Ryan Jonasson's wonderful side-by-side demo of Microsoft Virtual Earth and Google Maps.