My guess is -> on the internet
You're not mentioning which WM you're using or anything...currently, I'm using KDE, which can be made an orgy in transparency (easily set through menus/the control center). Some apps (i.e., XChat) have own transparency options. As for wallpapers, you might want to check out DeviantART
, for example. If you're using KDE or GNOME, you also might want to take a look at KDE-look.org
, although the latter has been a bit sleepy lately. Qui quaerit invenit.