Cyber-D's AntiScreensaver is a tool which will stop the screensaver from launching if you've viewing a certain website or running a particular program.
You might be tired of the screensaver firing when you're watching a video with VLC Media Player, for instance. But all you have to do is launch AntiScreensaver, click Add, and enter the text VLC. AntiScreensaver will simulate a keypress every 60 seconds just as long you're running an application with "VLC" in the title, so keeping the screensaver at bay.
You're not restricted to entering application names, of course. Add the text YouTube and the program will block the screensaver whenever you've got YouTube open in a browser window. Add other websites, file names or other text to suit your needs.
And an "off" button in the AntiScreensaver window means you can leave the program to work most of the time, then easily disable it whenever appropriate.