RedHat's Gnome and KDE are very similar in look (the Bluecurve theme). All versions of desktops - Gnome, KDE, OSX - look the same (IMO) because they are trying to attract Windows users away from 'the dark side'. This is on the grounds that if users want a particular look on a desktop, the alternative OSes have it.
(Just an opinion, no flames please)
On the other hand, the alternative OSes are customisable and can change their look drastically - which is a good thing - and can look very different to Windows. Take a look at themes on www.kde-look.org
or on www.gnome.org
(don't know the exact link for Gnome themes, sorry)