Proper

• Valve Software - Makers of Half-Life® and Half-Life® 2, as well as the Source™ engine.
• Havok - Makers of Havok Physics™, the physics engine used in Source™ games.

Source™ Related

• Valve Developer Community - The current primary resource hub for information about Source™, including articles, HOWTOs and tutorials ranging from design theory to level design and coding.  I'm fairly certain a lot of the Wiki's material came from the HL2World Knowledge Base, which from what I can tell has been inactive for over two years now.
• Interlopers.net - This is a great website geared especially towards Source™ engine developers.  Apart from supporting a robust forum (which the VDC somewhat lacks), it also has its own repository of articles and tutorials ranging from simple guides to advanced theory and design.  If you were only allowed to bookmark two Source™-related websites, they should be the VDC and Interlopers.
• VERC Collective - This used to be a primary information hub for editing Valve games, including the newer titles. Their Source SDK forums are no longer active (they were moved; see below), but this site still contains a wealth of information to be unearthed.
• Source SDK Forums - VERC's Source SDK forums were moved to the Steam Forums.  This is always worth a visit.
• Iwannamap.com - Another website that provides some Source SDK tutorials.  The tutorials cover an array of basics for beginners, as well as some pretty fun ideas (like making a Dance Dancer Revolution map).  Iwannamap.com also boasts its own forums.

Utilities & Stuff

• Nem's Tools - Nem has contributed so much to the developer community that it's only fair that he's listed at the top of the links here.  He's provided a lot of useful software tools to assist in making models, maps and textures.  On top of that, he's also co-authored some VTFLib, which is a nifty resource for programmers.
• wonderboy.org - "Jed"'s website, the other co-author of VTFLib and the creator of some very useful applications as well--such as Studiomdl and the 3D Studio Max SMD and VTF plug-ins.

Maya Stuff (It's my site so I get to make an entire section for it)

• Prall's Half-Life 2 Exporter for Maya - This is what you download if you wish to import or export SMD files using Maya.  Prall's scripts are included with the Source SDK, but last I checked they only give you version 2.4, while the latest (and final) version is up on Prall's website.  The scripts are MEL files, and can be a bit slow and buggy (especially with importing), but they get the job done.
• Palmer's Maya Prop Exporting Tutorial - Pretty much the de facto website that explains exactly how to install and use Prall's scripts.  It's a great tutorial and it also helps explain a bit about some of the quirks surrounding Prall's utilities (such as placing different models inside of layers, which so many people to this day still overlook).
• hohehohe2's OpenMaya Tutorial - Quite good documentation on setting up and using the Maya API.  A great resource for coders just diving into using the API.  If you do a little more digging on the main site you will also find some updated information on dealing with Maya's API on newer version, like implementing Python within your MLL plug-ins with Maya 2008.
• CGTalk's Maya Forum - A great place to go for asking questions about Maya, or reading threads to learn more about Maya.  It even has MEL scripting, Dynamics, rendering, and character rigging subforums!