This is a rather old but never before on Moddb released HL2 sp-mod that contains 4 large sp levels. it can also be played with HL2 ep1 and or 2.
You get extracted in the sewers outside a big city that is controlled by the combines, although there are certain parts of the city that are filled with headcrabs and other creatures. You have to find and fight your way trough the sewer, over the railroad track in to the city. Once you're in the city your objective is to find the power plant and destroy it. It sounds easy but you will have to fight for your life before you get there.
Although this can be called a sp-mod it is in fact in sp-mappack, the content is 4 maps without any custom content and therefore you just need to place the 4 levels in to your maps folder, (please read the readme.txt that comes with the download.)