http://deta12.com/hl2d.html
Apparently it is http://deta12.com/hl2d.php now.
Edit: Hmm, it crashes for me when I try to shoot the SMG. It makes for a good game of chase though. By the way, the GMArcade "Play Online" link didn't work for me either. But I'm on a public comp, so maybe that's just me.