S60 Theme

Symbian, mobile operating system that has one advantage over XP, they let you make themes.

There are a few great sources for S60 themes, of which google is NOT one, make sure never to visit a site that has "FREE" in it's name. Deviant art is probably the best site for GOOD or at least decent quality themes, although Symbian fan-sites like Symbian Freaks has a topic dedicated to themes, although quality does vary (Don't tell them I said anything).

IMPORTANT:
Now before you go installing themes, make sure you like it as you CANNOT uninstall any themes until Nokia patch the S60 software.

Before you run off and begin downloading themes, i'll put up a few of my preferred themes with a brief review of them. My phone is a N95, so these themes will only be compatible with 240x320 px screens.

Vista Media Center Theme
Worth a look, it follows in the long string of currently popular vista themes.
Pros:
Works relatively well with landscape
Has custom icons
Cons:
SMS and calender text is a pain to read, doesn't contrast.

HTC/Windows Mobile 6 Theme
The black theme works very well as a minimal theme. Although green has an issue with text contrast. So i'll review the black theme.
Pros:
Works perfectly, no stretching occurs in landscape.
Defined and easy to read, text contrasts.
Cons:
No customised icons.

iPhone alfa
Probably the best iPhone recreation I could find, the background blurs when you enter the menu which makes you feel like your viewing the foreground.
Pros:
Diverse background
Well created menu bars
Landscape blur works very well
Cons:
Fish look warped in landscape home page
Text badly contrasted in some places
Signal/Battery bar is absolute shit to read
Analogue watch background stays when you change the time setting to digital

Nokia N-Series theme
Your N95 may or may not come prepacked with this theme.
Pros:
Text is incredibly easy to read
Simple theme
battery/signal icons are easy to glance at
Cons:
Landscape home page FAILS.

Ayofe
Completely original, it'a a hate or love theme. It may take some getting used to, I found it strange staring at the colour scheme.
Pros:
Everything is easy to read and contrasted
Made from scratch
Works perfectly in landscape
Custom icons
Cons:
Icons look very similar, and are hard to distinguish
Battery and signal bars aren't easy to read.

Good luck finding non-windows themes.
Start with DA and see where it takes you. If you have any links to themes you want placed up here, drop a line by me.