Making Movies
Half-Life has the capability to record it's video out to a custom file. You can then process it with a utility from the SDK that saves each frame to a separate .bmp file, and compile it into an .avi with programs like Adobe Premiere or Fast Movie Processor. This is a great way to include footage from the game into a promotional video for your mod. Unfortunately no sound is recorded at all during this, although Total Recorder works very well for capturing the sound from Half-Life. Also, these files can get really big. It's around 70 meg for just 30 seconds of video, although you can make your final .avi reasonably small by compressing it.

Note that this is different than recording demos, which can only be played back from within Half-Life and are much smaller.


How-to
1> Run Total Recorder. Go to Options, then Recording source and parameters... and choose Sound board. Set the quality to CD quality at 22050 Hz. Then hit the record button.

2> Run Half-Life and bring down the console. Type startmovie movie. This tells it to start recording. I called it movie but you can name it whatever you'd like.

3> Type endmovie to stop recording and quit out of Half-life. Whether you were playing Half-Life or a mod, the movie will always be saved to the Half-Life folder.

4> Press the stop button on Total Recorder. Then save the .wav file wherever you'd like, maybe to the Half-Life folder.

5> Drag-n-drop the movie file onto mkmovie.exe. It will create .bmp files for each frame. It will give them the same name as the movie file, so if you'd like to have a different name, run it with the command line:
mkmovie [-basename <.bmp name>] <movie filename>

6> Load the bitmaps and wav file into Adobe Premiere or Fast Movie Processor. The recommended frames-per-second setting is 10 to 15, and recommended width and height is 320 x 240. Sound is recorded at 16-bit stereo, but you can drop it to 8bit mono to reduce the filesize. The quality will only go down a little. Cinepak Codec is a popular compression codec, although uncompressed will always yield the best picture quality but also yield the largest file size. Test out different compression settings to see what you like.

7> Save or render it as an .avi, .mov or whatever format you'd like.


Resources
Download mkmovie.exe to process the video file from Half-Life. I've compiled this from the Half-Life SDK. It includes this document so you can follow these instructions offline.
Go to the Fast Movie Processor website and download this video editing program. There are limitations with the shareware versions of each of these programs.
Go to the Total Recorder website and download this audio capture program. The shareware version only records up to 40 seconds of sound.