Colour Simulations is a free tool which can simulate - or help counter - various types of colour vision deficiency.
Designers could use the program to understand how people with common red-green colour vision conditions (deuteranomaly and protanomaly) might see their creation. Someone with those conditions might use it to help them see colours more clearly, and the program could also be handy to anyone who wants to tweak screen colours occasionally.
Colour Simulations works by displaying a movable, resizable filter over the screen. Right-click this and you're able to choose a colour vision deficiency, or colour transform you'd like to apply (Brightness; Contrast; Saturation; Greyscale; Invert; Circadian light block; Colour swaps and filters). Then spin the mouse wheel to maximise or minimise the effect.
This is mostly designed for occasional use, but you can also maximise the filter window (press H to make it hit-transparent, F for full-screen). Browse the web, watch videos, use other applications and their colours will be adjusted in real-time to match your settings.
Colour Simulations provides an interesting way to learn more about colour vision deficiency. It can also compensate for those conditions and other issues, although if you just want a colour filter for your screen there are simpler tools around.