Valve sends word that the Half-Life Software Development Kit is now available on this page for mod authors eager to try their hand at writing modifications for Half-Life (though as of this writing there don't seem to be any download links there). There's info on the page with what seem like very liberal restrictions on commercial exploitation of your work, and here's word on the two versions of the SDK they are making available:

There are two versions of the source release: the Standard SDK, and the Professional SDK. The Standard SDK is what we're releasing on the web for anyone to download. This is the traditional kind of source code release people are used to. It will contain everything you need to make a mod. You can make new weapons, new gameplay rules, bots, new entities, whatever you want. You will not, however, be able to rebuild the hl.dll file.

We're also releasing the Professional SDK to folks who explicitly ask for it. Most mods I've heard of won't need it, but for those folks who want to modify rather than replace hl.dll, you will benefit from the Professional SDK, with the complete DLL source code.

It's probably helpful to give an example. If you want to create a bot (the most often requested mod by my count), you will not need the Professional SDK. This is assuming that your design for bots would not use the existing monster AI code.

The Standard SDK will be available on-line and is 775Kb of zipped source code.

The Professional SDK is about 600 MB, and will be available by CD once you have signed the enclosed Non-Disclosure Agreement (NDA). The NDA says you won't turn around and give the contents of the CD away, and you won't make a commercial product without talking to us about it.