Oh god the nostalgia hurts. Why did custom edition have to die?
Coldsnap kicked ass - one-shotting a longsword with the mythos, or sneaking through the mountains and bombing the mythos with the longsword, hunting jets with the gauss hog, shooting down the chumps who thought it would be a good idea to make use of the pelicans... There was another really good one too; Extinction, I think. Good times.
But it didn't have an engine that permitted much expansion. Getting more than a few different vehicles and weapons on a map was a hack, there was very limited room for scripting, the visuals were really limited in quality, it wasn't very easy to learn and it didn't have a very big community, so once all the people doing cool stuff with it (i.e. Tiamat) moved on, there wasn't much left.