HL Sources
If you want to download executable files or compiled mods instead of source code of them, here's the instruction how to do it for Github.
First - make sure there is no "Official website" link exists on HL Sources page, which would look like this:
If there's no such link for your item - then go to github page of selected engine/mod (using main blue link), and then click on the link from the "Latest releases" section:
This will open page with latest release. Click on one of the files for your platform (like win32 / win64 etc.) in Assets section to download it (choose one without Source code / Source / Src in its title).
If there was no "Latest releases" section on the right column of github page from previous screenshot - it means there are no official releases on github yet, so you will need to either find compiled releases elsewhere, or compile it by yourself.