On customizing a Mac

Didn't think interest in this would come back to me, but I would like to shake things up on my computer, visually speaking.

Background: I remember going nuts over Whistler (Windows XP) UI leaks from early builds. The first time I saw the Luna visual style I went nuts. Bill Gates revealed XP and I decided I absolutely needed my computer to look that way. I was running Windows ME back then, and am glad to report I was not plagued by the horrible instability most people seem to have encountered.

I remember waiting on an IRC chatroom with terrible anxiety for the imminent first leaked RTM (infamously uploaded by devils0wn with the FCKGW key). I had to queue for hours to get a download slot. It was crazy and I loved it but it wasn't long before Stardock made changing my UI a breeze. Much of what I achieved would make my eyes bleed today, but at age 12 aesthetic appreciation is a little different I'd say.

I remember reading a "Post your Wallpaper" thread here where some users rocked very nice looking screens. I have no idea how they did it or how these tweaks are called but they seemed like modern-day variants of an active desktop: minimalist calendars or weather widgets right on the desktop.

I am curious and would like you guidance. I last modified Windows system files for theme modification sometime around 2005 and have never messed around much since I jumped to OSX. I'm hoping some of you guys can point me in the right direction! Instructions, screenshots of what your desktop look like are all welcome :)

TIA. For the TLDR crowd: tips/examples of Mac desktop UI customization please?