FTE and DarkPlaces both support customisable particle systems (though I admit I don't know much about DP's).
These particle systems allow you to customise existing effects so they suit your mod better. Alternativly they may (at least FTE does) allow you to register your own particle effect types, or just override specific uses of the particle builtin. This would allow you to make someone else support/maintain the engine for you, which is a _HUGE_ time saver.
If you wish to go for an existing engine with a lot of custom effects already, telejano probably has the most. And 90% are so increadibly specific to something that you'll never find a use for them. Darkplaces has several additional particle effects also. FTE had a customisable particle system earlier in it's development, so there are less additional specific tempentities (having said that, most of DP's are supported, they just don't have effects defined yet).
Plus it might help to use an engine with skyboxes.

Anyway, now that I've failed to get you to use an engine that someone else is responsible for, CL_ParseTEnt in cl_tent.c would be a good place to start looking. As well as in protocol.h for the defs (though that's less interesting).