Basically it's a free mod that's been several centuries in developement but updates the original Half Life to the UPDATED Source engine. A labour of love and much much better than the official HL:Source.
I haven't had a huge amount of time on it but for free, it's astonishing. Revisiting the old story again brings back so many fond memories - it's truly an engrossing game and I may find myself playing the whole series again .
MOre impressions when I can but sure many of you have already got it.