I use Blender, a free program that can do modelling, uv mapping, texturing, animation, rendering etc. really a great program.
www.blender3d.org is where you can get it and some tutorials (including video tutorials).
Wierdest thing I did in HL1 was the first time I played it I managed to jump out the back window of the tram and ran around the tunnel. I got down to the area where you see the apache on the landing pad, I went up to the big hangar doors but they wouldn't open :( Then I had to kill myself.
At the moment there aren't exporters from Blender to HL2, but I'm pretty sure that there will be one in the near future (Doom 3 one was written quite awhile ago now, HL2 is bigger so here's hoping). Though Blender can export to .obj files etc. so as long as that can be used to get a model in HL2...
Blender is a free program that does modelling, texturing, uvmapping, rendering, animation etc. www.blender3d.org is a good site, you can download it there and also see some video tutorials (strongly suggest watching them if giving the program a go). www.elysiun.com is where most of the community...
Some of mine:
http://www.applepics.com/6/userfiles/417fdb67d361e.jpg
http://www.applepics.com/6/userfiles/417fdb7d0b2ac.jpg
http://www.applepics.com/6/userfiles/417fdb8aa01cc.jpg
http://www.applepics.com/6/userfiles/417fdb98d35d6.jpg
http://www.applepics.com/6/userfiles/417fdba7e6e9d.jpg...
Hehe polys. The reason he doesn't have fat is he doesn't have anything over the layera of muscle :) I was doing this so I could work a lot of detail into the model and make it very organic, as there are many muscle groups.
It is pretty much just a model (incomplete so far) of human anatomy...
Someone on the Blender forum I visit made a muscle shader which they put up for download. I tweaked it a bit and added it to a mesh I'm working on. It is obviously not perfect, but it is quite icky :)
Just takes practice :) The Joan of Arc tutorial shows a good way of modelling the head, and shows good edgelooping (going with the contours of the face).