There are several sites providing "themes" that you can apply to OS X. You do need an app to apply them. Check out this Gettng Started page page to download a theme changer (I usually use ThemeChanger.
One site of a guy who does very nice work is Max Themes (where the above getting started link is at). Here is his Themes page. Check out his "brushed" theme. I like Smooth Stripes Glossy, myself. You can even sully your Mac desktop with a Win XP theme, if you want.
Another collection of themes -- and LOADS of other ways to hack your Mac -- can be found Res Excellence. There are OTNS of things you can do here to personalize your Mac: boot images (replace the grey Apple), boot windows, login windows, desktop pics, icones and MUCH more.
Keep in mind that hacking your Mac can be risky business. Having said that, I have done it for over a year now, and have yet to experience a problem (touch wood). I have submitted a few of my dekstop pics to Resexcellence (there's a user gallery there). The site is down at the moment.
You can really change the look of your Mac this way. I've done smoother, BETTER looking Aqua themes, industrial looking themes, and full-blown "Matrix" themes. Not only with theme changes, but also new icons sets using Candybar and dressing up the Dock using Skin-A-Dock. Skin-A-Dock does make the Dock a little sluggish, however -- at least on my humble G4/450.