The info_notnull explosion is a classic, and it uses just one field on one entity to work. The trick is to take an info_notnull, and add a key:

use barrel_explode

Then give it a targetname and a trigger that targets it. When the trigger is fired, the info_notnull will explode. When an entity is targetted by a trigger, the QC calls whatever function is in the use field of an entity. Most entities like doors and triggers set this field when they are spawned, overwriting what you might want to add. But an info_notnull has no code in it's spawn function, so you can add any function to use, and it will be called.

barrel_explode is quite a powerful explosion, liable to injure or kill the player. A nicer, less damaging alternative is OgreGrenadeExplode which does the damage of, you guessed it, an ogre grenade. tbaby_die2 is fun for a wierd effect, it's the purple explosion of a tarbaby.

Unfortunately all these functions end up removing the entity after the animation has completed, so the only way to get multiple explosions out of one of these is to trigger it more frequently than one every 0.5 seconds. As soon as you let it fully animate once then it's gone. There may yet be a way to get repeated explosions from one entity, but I've yet to hear it.

If you root around the quake source, you can find other functions that do things besides explosion effects, and many of them can be performed from the use field of an info_notnull. In addition, any other entity that doesn't have anything in it's use field by default(eg ammo) can do this same trick and explode on a trigger. Exploding ammo probably isn't the best example, but I'm sure there's some useful example of the idea.