What about a nice thread for HL1 mods? I'll start with two recent ones I loved (Around 2012, which is recent in HL modding).
One is
The Trap, it plays kind of like Portal, but the focus is completely different, it has different puzzle elements, and the execution is flawless.
Highly recommended for anyone into both HL and Puzzles.
Link:
http://www.moddb.com/mods/the-trapThe other one is
Hazardous Course 2, which is the best HL mod I've ever played without a doubt. Don't let the name fool you, it's not just a hazard course, it's an EXTREME version of the original course in every single way conceivable. Absurdly big, balls to the wall hard, full of secrets, the best level design I've ever seen in GoldScr by a huge margin.
Definitely a must play for anyone looking for the ultimate Half-Life challenge.
Link:
http://www.moddb.com/mods/hazardous-course-2PS: You can find all the secrets in any order so you don't have to worry about forgetting some, you will see what I mean, it's part of the marvelous level design.
Also the mod has some bugs, which might be frustrating, one of them is in the train section, if the train gets stuck you have to save and load, it's rare but it may happen.