Poly9, a group of devolopers based in Quebec City, recenty launched a web-based 3D Earth product called FreeEarth that requires no download. Free is pretty slick and I would love to see FreeEarth as an embeddable widget!
A Flash-based application, FreeEarth, offers an open API that can be leveraged to create some interesting mashups. If you a twitter fan check out twittervision3D. If you show Flickr love then you might want to check out flickrvision.