Minimicus was released in 2003. Despite its tiny size (pun intended) it is undoubtedly a classic Half-Life mod.
You start out, full sized, in a lab and have to minimise yourself in order to proceed. The authour has done a really good job of making the world seem huge in comparison to the player. The gameplay is very original and a lot of fun. The attention to detail in maps is good, with some fun little surprises dotted around. My only complaint is that it wasn't long enough!
Like I said, Minimicus is without doubt one of the classic HL mods. Download and play it, now!