Screen Mode Widget! It's free and will toggle your screen between auto sleep, auto off, and always on.
I also use Screebl and consider it a MUST HAVE. Screebl will keep your screen on as long as you are holding it and when you lay it down, the screen will go off. You can set preferences of varying angles and screen timeouts, but I just use the defaults and it works perfectly. I bought the pro version to support the developer, but the free version does exactly what it says.
I only need to use Screen Mode widget if I want to override Screeble and keep the screen on when it's lying flat.