I have never really used terrain in my RPGs, except purely as scenery. For those of you wanting terrain to play a bigger part in your encounters, Terrain Toolbox, from Sneak Attack Press, gives a lot of options.
Terrain Toolbox is a 32 page PDF, with the first part taken up with an introduction, and an overview of how to use terrain in encounters, and a specific set of rules. Each terrain is considered a level, which measures the effect of it. There are some mathematical oddities in the rules , for example, a terrain can do low, medium or high damage, but a terrain with low damage at level 1 does 1d8 +2, while a terrain with high damage does only 1d6. At higher levels the damage numbers start becoming what you would expect.
The bulk of the book is taken with terrain examples. They are extensive, and useful to almost any game, with plenty of variety, and suggestions on how they could make an encounter more interesting. The emphasis is firmly on their use in a combat encounter, and a discussion of other ways to use terrain would have been very interesting… Perhaps in a future supplement?
The layout of the book is very good, and it is easy to find the relevant terrain. It does not use any PDF features, but with its short length it does not really need any.
The Terrain Toolbox is available from RPGNow for $4.95, and is perfect to add depth to combat encounters, preventing them from getting stale.
This review is for the Pathfinder version of Terrain Toolbox, there is also a version for 4e available.
Dislaimer: This is a review of a free copy the I got from RPGNow.