Is there a way to kill yourself immediately?

If you are playing SMP, December 1st, 2010 update removed the /home command, and December 3rd, 2010 update added a /kill command, which will kill you upon issuance.

In SSP, there is no 'immediate' way to kill yourself without some additional setup, or handy terrain.

Just stack about 25 blocks underneath you by jumping & placing a block underneath you (at the height of your jump). After your stack is built sufficiently high, simply step off the edge to your death. This stack can be made of dirt, sand, or any block which you have an excess of at the moment.

Once you work out how many blocks you have to stack before the fall will kill you, it will only be a sub-second fall before you're dead. Also, the stack itself should only take about 10 seconds to build.

Tip: Use shears to cut leaves from trees, and build your stack out of leaf blocks for a self-destroying structure (the leaves will slowly disappear over time) in order to prevent your world filling up with random columns of blocks.

Lava will also do the trick if you have some nearby.