Black Mesa Source
I swear I thought I posted something years back about this (the darned thing still isn't completed yet) which I was going to reference here but I can't find it. There is a new trailer on YouTube for this total conversion of the original Half-Life using Half-Life 2's Source Engine. The trailer contains some gameplay. The official website is: http://www.blackmesasource.com/. Can't wait for it to be released.
It certainly looks very good!
They should have done a similar thing for Half Life: Source -- that wasn't too "enhanced". Is that what this is? I only had a quick look, or is it an "opposing force" sort of thing?