The sound design in HL2 is amazing, and the Strider cannon sound is one of my favorite, not done justice at all by my pitiful attempt at assigning it letters with my keyboard. Wav files here: (charging) (fire).|
The visual effect is really neat too, which doesn't really come out in the screenshot. It looks like the cannon isn't so much firing as it is plucking a strand of, I dunno, space-time, I guess, drawing it back as one would a bow string, and then releasing it. It's neat warpy effect, though you don't usually get a good look at it up close while you're playing since it would mean your death. You can see it in this YouTube clip. You can actually freeze the strider while it's firing (type ai_disable in the console) to get a nice long look at it.
Comic notes: the beam in panel one is actually my G-Mod posing tool's beam, not the strider cannon, since I couldn't get the angle quite right. Explosion in panel 3 courtesy of about 7 SLAM mines. Love dem SLAMs!