Most (well-behaved) apps that need this (Keynote, Powerpoint, VLC, Quicktime, DVD Player) will tell the system not to go to sleep when they're doing something r

Most (well-behaved) apps that need this (Keynote, Powerpoint, VLC, Quicktime, DVD Player) will tell the system not to go to sleep when they're doing something relevant. Caffeine is for when you're doing something like watching Hulu, but even then you can enable hot corners in the system preferences to handle this for you. VLC (at least in the past) has been kind of a dick about it; it tells the system not to go to sleep or blank the screen whenever it's running, whether it's playing a video or not. Dunno if they've fixed this yet.