Prefabs.

Once upon a time, many years ago, you could Google (or perhaps Yahoo back in those days) Half-Life prefabs, and a number of websites would come up giving you all the prefabricated constructions you could want for your Gold Source maps. Not anymore though sadly. Even the once great Gamedesign Prefabs has fallen into ruin. Fortunately, there are still places to turn. A slightly deeper search on Google turned up the following sites:

• Snarkpit - Featuring 107 prefabs. Quite a mix of stuff on there, making it worth a look through. Lots of vehicles as well, which are always a pain to build.
• Game Banana - 31 on this site. Some seem to overlap with Snarkpit, but there are a couple that appear useful (Black Mesa train perhaps?)
• File Planet - Contains 16 files. Some handy, some not so handy, however the best by far are the PrefabLab archive. I have this unzipped on my hard drive, and I look through the pictures for what I want. Worth going deeper though as not all files have pictures. Well worth the download and time.