Free Lightroom Presets
Heather Green has created a list of free Adobe Lightroom Presets.
I have not tried them all, but they are great examples of how much you can do to modify how your pictures look before needing to use Photoshop. I personally really like presets from Matt Kloskowski's Lightroom Killer Tips.
Great list Heather, thanks for putting it together.
UPDATED: new link from Heather: http://www.heathergreenphoto.com/blog/2008/2/2/adobe-lightroom-free-presets.html