In Vitro is a free Windows 7 Theme, which makes almost everything fully transparent on your Windows 7. You can see through almost all the windows, have a look at these snapshots below.
Yes, it gives full glass effect. The download links for In Vitro can be
found here. Go through the instructions found on the page and after you extract the contents of rar file.
Update : In Vitro Theme is no more available, Please find the following theme which is very similar to the above theme.
Glass Onion Theme
Aero VG Theme