Suppose for example I am interested in creative ways people have made trees, or minifig scale furniture, or just architectural details like windows. Is there some encyclopedia that organizes pictures this way?
The Lego Ideas Book
Recently released last month (19th Sep 2011). Written with the help of adult Lego Fans. I've had a look through the book myself. The book has six chapters focussing on transport, buildings, space, kingdoms, adventure and 'useful makes'. The buildings chapter in-particular focusses on every day objects.
The photography site has a large community of Lego Fans who share there creations. There is an entire group dedicated to Minifigure scale furniture.
If you can go, try attending an AFOL convention. Brickcon is always a great source of inspiration to me. You'll see great examples of everyday objects & building styles & you have access to some of the model builders themselves. You can get helpful hints from them that even the excellent answers mentioned above can't provide.
You can visit the website buildingexamples.com A lot of examples are there with step-by-step instructions.
You can also get ideas and suggestions from the Rebrickable website. You can see what others have built and get instructions for them.