So, in searching for sounds to use with my dice roller for the Ghostbusters RPG that I released yesterday, I came across a couple of links, and since I'm in the blogging mood thought I might share.
The HProps website has some of the best sounds for ghost busting equipment, and is certainly no secret amongst the Ghostbusters fan community. While I don't think I'll ever actually build a Proton Pack, two of my players have put these sounds on their smart phones, so when they want to flip a switch that makes a worrying hum they actually can. If anyone wanted to use them for GB Roller, you'll need to convert them from MP3s to WAV files, but if you haven't got a solution for that already Audacity (http://audacity.sourceforge.net/) is free and easy to use.
That's all I've found (so far) for Ghostbusters specific stuff; the next two links are good for GMs of all systems and genres.
SoundFX Now! offers pages of sound effects, some more useful than others for GMs (the machine & technology and weapons & war sections might be the most popular, but the other sections have some hidden gems too). What's good about this site is it offers the files to download if you click the title, but also presents a page full of effects you can play straight from the browser.
Probably one of the better known sources of free sound effects, though I generally find that, because of its open nature, there's a lot of content there, and you've got to really hunt for the good amongst the average and not-so-good.
Okay, that'll do for now. If anyone's got any suggestions of other sites they use, or wants to point out awesome software for playing said sounds specifically useful to GMs, comments are, as always, open and welcome.