We talk a lot about tweaking your user interface around here—often inradical ways—but if you'd just like to make a few small tweaks to Windows Aero, free app AeroTuner gives you a few more options than the Control Panel.
AeroTuner won't tweak so much as to make your Windows 7 UI look unfamiliar to other Windows user, but it certainly adds some cool customisation options to the mix. From AeroTuner, you can set the two main colours of the Aero window, take away transparency, and tweak things like Aero Stripes (which essentially add more of a "reflective" look to Aero windows). While you can always tweak Aero from the Control Panel, AeroTuner gives you a few options you don't get from the built-in customisation tool—so if the colours in the Control Panel are a bit too bland, or Aero is just a little too transparent, you can fiddle with a few more settings to tweak it to your liking.
AeroTuner is a free download for Windows 7.