You can argue whether you find these applications essential, but because my employer has an island in Second Life and because the number of KML files online is growing rapidly, I use Second Life and Google Earth a lot.
Second Life
Second Life is notorious for its instability and frequent crashes on my work laptop. Luckily it runs stable on the Macbook. Because the graphics cards lacks power you can’t enable all the advanced graphics options. But voice is also available on the Macbook.
Using Second Life on the Macbook took a bit getting used to because on XP I knew how to use a lot of the keys to fly etc. and since here also the Ctrl and Command keys got switched I look a bit like a noob from time to time when I use Second Life on the Macbook.
Google Earth
Great application to display geotagged pictures on, display GPS tracks on, or Wikipedia articles. And now you can even fly using a plane (try Command+Option+A).
Stellarium
Stellarium was around before Google added the stars in the sky to Google Earth and even though I know little or nothing about stars, it is really cool to just select a position on the earth, set a date and time (or just use the current date/time) and see how all those small bright spots in the sky are called.
Stellarium is available for Windows and Linux also, so even without a Macbook you can enjoy this great free application.