Yogg Saron. Raid-wide Sanity monitor. Need I say more? Download here!
I was quite happy when I found this Grid addon. What it does is pop up a sanity alert icon on the raid frame when that player drops below 50 sanity (or you can edit it in the Status menu to be a different threshold), at which point you can remind that player to get their tail over to one of Freya's sanity wells before they go all "Do you read Sutter Cane?" on the raid!
I tested it out this past weekend; the only setup it requires is that you decide a location on your Frames to have this alert pop up. Grid -> Frames -> (choose location and put a checkmark next to "Sanity" for that location). If you want a different threshold than the default, just alter it in the Grid -> Status -> Sanity settings.
I recommend putting it in a different frame location than your other Yogg Debuffs, such as Squeeze and curses/poison/magic/disease, so that it isn't hidden beneath those other statuses or otherwise conflict with them. For additional icon places for your frames, you can download CornerIcons or SideIcons.