(UPDATE #4, 4 January 2011 -- added Twister and FontForge, check the "Special Teams" section... and a note about using D3 / Q4 textures)
THE BASICS (stuff almost everyone needs)
- Nem's GCFScape lets you extract files from Valve's .GCFs as well as .VPK files introduced with the Left 4 Dead games. (GCFs were too prone to file fragmentation, so it looks like Valve's shifting towards a bunch of smaller VPK files.)
- Nem's Photoshop VTF export plug-in. 'nuff said. While you're at it, grab his VTFEdit too. If you don't have Photoshop and aren't willing to acquire it, then get GIMP and Tom Edward's gimp-vtf plug-in. (Fun fact: celebrity modder Adam Foster used GIMP for texture creation for a really long time, so it's certainly up to the task and a viable alternative to Photoshop.)
- Jed's VTF Shell extensions / Thumbnail display plug-in for Windows has saved several lives.
- Notepad++ and Tom Edwards' VDF add-on. Makes editing soundscapes, soundscripts, and other resource / script files a lot easier.
- Mod packaging checklist. No, seriously, use this. A professional presentation goes a long way.
SKINNING / MODELING TOOLS
- Propper is a modded version of VBSP that lets you convert BSP brushes in Hammer directly to static props or physics props, along with extensive support for multiple skins and LoDs. Great for the lazy level designer in all of us; this thing is amazing.
- There's an updated version of Jed's original 3DS Max export tools for the Source Engine by Shawn Olson. It's mostly an updated MaxScript GUI and it looks really helpful. "Why didn't we have this 3 years ago?!"
- Blender is a free open-source modeling / texturing / everything editing package that has good Source SMD Tools. Many artists dislike the interface, however.
- There's also a free (but rather old) Softimage XSI Mod Tool that
no one ever used / failed miserably to get a user basesome people use in the mod community. It has native SMD export to Source (I think? I only used it twice) but it's based on the old Softimage 7.5 codebase. In general, a lot of people think it's all gone downhill since Autodesk bought it out to destroy it.
TEXTURE PACKS, MODELS (remember, always credit the author!)
- Almost everyone in the game industry uses photo source from these two free and awesome sites: Mayang and CGTextures. (Both are awesome, but if you're gunning for a career in the game industry then you should definitely paint / modify your textures more, so that an art director looking at your portfolio won't recognize that unedited tiling brick texture you lazily threw into your map.)
- PhilipK's futuristic metal space station "PK" texture sets (scroll to the bottom) are really awesome, but can use some VMT tweaking to get the effect that you want.
- Hipshot's cliff textures, great for displacement terrain. I think the alpha blends are already setup.
- Hipshot's (now overused?) skyboxes: Grimm Night, Miramar, Stormy Days, Violent Days, Interstellar.
- Snarkpit's prop model database. Very old, but there's some really great stuff in there.
- TF2maps.net prop model database. Most of these models could easily be re-textured for a more photorealistic mod or level.
- Kathar's prop models. Mostly intended for DoD:S maps, but works for everything. Cows, farms, trees, etc.
- Warby's dystopia prop pack. Giant spider robots, futuristic laser fences, tech furnaces, etc. All expertly crafted.
- In general, many custom Doom 3 / Quake 4 textures can be easily converted to VTF / VMT files. If you see stuff in a level you like, just e-mail the author for permission to convert & use them in your own project, re-name the .pk4 to a .zip, and then export the .tga files through the texture authoring program of your choice. (see "Photoshop" / "VTFedit" / "Gimp" bullets above)
- Microbrush 2 is "a BSP editor on steroids." Make brushes do crazy things that are impossible in Hammer.
- DispGen allows you to generate displacements from height maps; paint a mountain, and it'll build a mountain for you in Hammer. You can even grab real topographical data from the USGS and make displacements from that. (R_Yell used this a lot for his amazing huge open levels in the Empires mod and Fistful of Frags.)
- Twister does crazy things with displacements, like twisting hallways into vortexes. Awesome!
- There are a number of Source batch compilers: Nem's Batch Compiler and Quicksilver's Batch Compile Tool. Personally I preferred Nem's for HL1, but now neither works particularly well because Source does map compiles strangely to interface properly with GCF files. (And I question whether you even need a batch when most BSPs take (at most) a few minutes to compile on the average mid-level machine. Nonetheless, they have their use if your map takes > 5 minutes to compile.)
- If you're working on a huge mod with more than a few team members, you'll want some kind of version control software; on Black Mesa Source we used Subversion with TortoiseSVN (and maybe should've interfaced it with WinMerge which apparently does file diff a lot better.)
- If you want custom HUD elements, you'll have to do some font editing with the free open-source FontForge. (Valve uses vectorized font files so they can be resolution independent.) However, if you just want a fancy menu logo, don't bother with a custom font -- you can just use the game menu logo VTF support that came with Team Fortress 2.
Please leave links to new resources / broken link complaints in the comments.