This invisible avatar skin from Cloud City (Blink Teleport Here) is technically a skin, so it's worn as a Body layer rather than a Clothing layer. It's available for both male and female avatars, and you can even use this base to turn your avatar into a skeleton using other Cloud City parts, like a couple of our Monster Ball attendees did. The invisible avatar skin from Cloud City is by definition a pretty tricky piece to photograph, so I've tried to make it a little clearer by taking most shots with my cursor hovering over my avatar to give the silhouette a clearer outline. Even so, as the above shot illustrates the avatar itself is 100% transparent except for my hair and clothing.
It reminds me somewhat of Second Life's "alpha layer", one of the more recent innovations for SL's avatar. With an alpha layer, sections of the avatar texture are cut out using an alpha channel in Photoshop that is saved onto a texture. When applied to this specific type of layer, the shape of the alpha channel is literally removed from the visible area of the avatar. Before the advent of the alpha layer, invisible prims were used to hide sections of the avatar in a slightly more awkward but no less effective way. This technology is useful for more than just making an invisible avatar, though. In Second Life, alpha has a few common uses, such as removing sections of the foot so that shoes will fit better, cutting off limbs so they can be replaced with things like peg-legs or robot-arms, and blanking out areas of the body to fit elaborate avatars like realistic animals.
This is not exactly the perfect avatar if you feel like sneaking around and spying on people, since your avatar name will still show and you will always have to have at least one item of clothing on over top of this skin. No matter what your avatar will wear an underclothes layer, so there will always be a trace of someone floating around, even if it's just a pair of boxer shorts drifting along the streets of Beach City. Floating underwear aside, I can't wait to see what other ingenious uses content creators will come up with for avatar invisibility in Blue Mars.