Instructables has an article on how to turn inexpensive hardware store caulk into casting material. I can see o-so-many applications for this one.
Modeling
Building African Terrain
Here’s a great illustrated post on building a spectacular African savanna battle board.
Modeling Birch Trees
The I See Lead People Blog has a photo tutorial on modeling the distinctive Birch Tree.
Making 15mm Littoral Terrain For DBA
Bob’s Miniature Wargaming Blog has what I think is a very neat construction method for making littoral terrain for his DBA Viking Army. The littoral is the part of a body of water that is close to shore.
Modifying Heroscape Terrain
Wargaming Miscellany has a post on modifying Heroscape hex tiles.