Google Earth has been about for quite a while now, so most of you will have seen it, I think... But Year 8 were asking about it today so...
You can download it here - Google Earth. In order for it to work, you will need a decent internet connection and a decent graphics card (I don't know much about this, but computer whizzes can check out the technical details here!)
Hours of fun can be had just flying about, but if you want to go on a sightseeing trip, have a look at Google Sightseeing.
Typing Google Earth into a search engine will come up with lots of links to blogs and forums (fora?) where you will be able to find a multitude of tips and ideas, as well as being able to download tours (for example, you can go on a tour of the Da Vinci Code locations...) and various overlays (earth at night, and active volcanoes are good ones!).
Have fun and let us know if you find anything interesting on your travels!