Minecraft wiki entry for Mechanisms, associated text: Lay TNT under critical areas of your base, about two blocks deep. This will prevent torches and other redstone contraptions from detonating your system early. Then, lay TNT to the surface in a 1x1 area and add a lever or button above it to complete the basic self-destruct system.
In creative mode, one could use dispensers filled with enderman spawn eggs wired to a 5-clock redstone circuit to release endermen into your house, slowly assuring a semi-complete destruction of your base provided that it is made from blocks that enderman can manipulate.
Remote dispensers could be wired to release fire charges upon your wood structures.
Pistons could be placed to release the floor below sand walls to quickly remove the walls of a structure. This can also be done by supporting the sand with scaffolding and breaking the main support scaffolds with pistons.
A dispenser with a lava bucket could be used to flood your base with lava.
Connect a mob farm to your base, and open it up. The mobs will cover your house. Creepers will detonate whenever anyone unwanted comes by, and the other mobs will fight and probably kill any intruders.
Use command blocks to really wreak havoc upon any sensitive areas. Use commands such as /fill, /summon PrimedTnt, or if you're really thorough, use /summon ~ ~ ~ wither. WARNING: These methods can and will destroy your base and anything near it.