A project a few weeks in the making (mostly was a file space issue), I have extracted and rebuilt every Left 4 Dead 1 and 2 beta build (specifically, the pre-release stuff), and I am now presenting them here, for archival purposes, and for ease of use.
They're all here on a nice little torrent file (So you can choose what build you want. Full collective is 126gb, so I don't recommend getting the whole thing at once).
I've included the contents of the torrent in the image above, for transparency.
(There's multiple post-launch betas I'm not touching, because I don't know if that falls under piracy or something, since they're technically the finished games, just with extra stuff. To be fair, I'm not even sure if this is breaking any rules as-is, so if it is, please let me know, mods).
The torrent can be found here (again, mods, please let me know if this is against the rules. Doesn't seem like it, but it might be. If it is, I can delete the link): l4d_betas.torrent
Please note that, while I have tested a few builds, I haven't tested them all. If there's any issues with any of them, let me know, and I'll try to find a fix/solution.
(For the L4D2 builds, they might not launch properly. I believe it's just a Steam App ID issue. I recommend you edit the Steam App Id file (generated after first launching it, should be found in the root directory) to read as 550 (L4D2's retail steam Id), and set the file to read-only. L4D1 should work, but if it has the same issue, just do the same thing, but replace the Steam App ID with 500.)