Everyone probably knows that a western-facing room gets warm in the afternoon. How much does that affect your AC costs? Obviously you've got the sun heating your walls, which are then carrying that heat into your home air. Planting trees on the west side of you house can save you a significant amount on your summer cooling costs. Here's the original blog article.
The coolest thing (pun very much intended) is that if you go with leafy trees, they'll drop their leaves in the fall and allow the western sun to slightly warm your house in the winter.