I didn't realize how powerful Android was in terms of customization up until earlier this week. I'd been using my X10 fairly lightly and running nearly all stock software and only downloaded some apps from the Market such as games and stuff.
Sony recently released the 2.1 update and I enjoyed my phone a lot more and started looking into some more widgets and stuff and found some really amazing stuff.
You can customize your home screens with themes and stuff with something like ADW Launcher or LauncherPro. This replaces the stock home screens and you can put a different skin and it looks impressive. ADW is free but many of the themes are not. There are some that are free though and they look decent. I'm using York's White and really like the clean look of it.
I was using Sony's media solution, Mediascape. It looks beautiful but in terms of usability it's a piece of shit. I was using it for several months but I finally got tired of it. There are some impressive music players on the market, including Winamp, PowerAmp (not free), Mixzing (free but there is a paid upgrade). These players have the ability to add a widget to the home screen so you can control the music without going into the application itself. Since I use my phone as my music player when I work out, I didn't want to have to always take my phone out, unlock it, go to the application, etc. just to change a song. I found a really cool app called WidgetLocker. It essentially puts a lockscreen in front of your normal wake up screens and allows you to put widgets on it. That way, I can just wake my phone up and change the tracks without needing to unlock my phone. The problem with WidgetLocker though is that it cannot get rid of your old lock screen, so you will have to brave three screens (if you have pin lock set up) in order to get to your phone.
In terms of calendar solutions, I've found Jorte, which is a free calendar/task app with a decent, if a little ugly widget. The nice widget I'm using is called Pure Calendar and Pure Grid (both are not free). They are just widgets so you need to have a backend calendar app, either Google or Jorte will suffice, and then you can use those widgets to provide an agenda and or calendar right on your home screens. A must have if you are using your phone for business/school purposes.
In terms of clock/weather widgets, Beautiful Widgets is really nice as well, with lots of different skins for you to choose from.
I've found a battery indicator called Battstatt that I really like as well. It puts out in a literal string your battery percentage. It's simple and doesn't have a graphic, so it's a clean solution. If you prefer to have your time like that as well, there's something call Tajm as well. I don't like my clock displayed as "Five minutes after six" though.
If I have some time I'll figure out how to post screenshots of my phone's desktop.