Star Wars TC
As far as conversions go, this is it. Star Wars Doom is quite possibly the most famous total conversion that there is. New weapons, graphics, sounds, enemies, the works.
There a four scenarios to chose from. The first, rather unoriginally called 'Star Wars' contains the main Star Wars levels, mostly set on Deathstar (I think), with all the new graphics etc. The second is called 'Invasion', and I would guess is meant to be the Star Wars characters invading Earth, as it places the new characters in familiar Doom landscapes. The third and fourth are scenarios from the original Doom, with the standard enemies replaced by characters from the Dark forces...
The sound effects are excellent. The weapon sounds are very 'ray-gunish', which certainly fits in with the theme. There are even a few samples from the film, for example when you die you hear Darth Vader say "Feel the power of the darkside". But I think my favourite one would have to be the Chewbacca groan which you hear when you kill an imp.
As far as the new levels go, they aren't bad, but not exactly the most revolutionary Doom levels I've seen. By trying to make the first few levels look as if they are set in the Deathstar, they have been made almost completely symetrical, which I think they must have found a little restricting.
Overall: So, Is it worth downloading? Well I think so, even if it's just for the novelty value. The levels themselves aren't bad, but the graphics, weapons and sound effects are very good.