ETC 2 is yet another single player escapade. It is quite long, about 3-4 hours, and has a(nother) good storyline. You have to fight off hoards of grunts and you get to use a few mounted machine guns and take out a tank. The level design is superb and it can be a bit puzzling in places. At the end you have to set off a nuke on Xen, to create a diversion before the scientits at Black Mesa try to close the remaining portal to/from Xen to earth. A good mod all in all. Not an essential play, but if you can get it, why, play it!
More info and download available at ModDB.