<p>Given a source color of any hue by the system or user, I'd like a simple algorithm I can use to work out a lighter or darker variants of the selected color. Similar to effects used on Windows Live Messenger for styling the user interface.</p>

<p>Language is C# with .net 3.5.</p>

<p><strong>Responding to comment:</strong> Color format is (Alpha)RGB. With values as bytes or floats.</p>

<p><strong>Marking answer:</strong> For the context of my use (a few simple UI effects), the answer I'm marking as accepted is actually the most simple for this context. However, I've given up votes to the more complex and accurate answers too. Anyone doing more advanced color operations and finding this thread in future should definitely check those out. Thanks SO. :)</p>

You can convert your color into the HSL color-space, manipulate it there and convert back to your color-space of choice (most likely that's RGB)
Lighter colors have a higher L-value, darker a lower.
Here's the relevant stuff and all the equations:
http://en.wikipedia.org/wiki/HSL_color_space
Another method is to simply interpolate your color with white or black. This will also desaturate the color a bit but it's cheaper to calculate.