Get them from the Fish Market, they're great when they're fresh
Anyway, it depends on your board's niche/target audience. For example a Computer Related board might need Programming Languages BBCodes, a movie site might use YouTube BBCodes, a music site could use mp3 Player BBCodes.
General/Popular ones would probably include strikethrough, media players and many others. Just do a search of the mods section and see what comes up.