One Way Glass
One-way glass
A clever mod that takes most of the vanilla blocks and turns them into one-way windows. Transparent on one side, but opaque on the other, these glass blocks filter light, turning a wall that appears solid into a massive window that can be used to survey the land.
One-way tools
This mod also comes with glass tools. Simple to craft and essentially infinite, they're a good tool to have that also comes with the advantage of being invisible to anybody but the wielder. Alas, these tools are made of glass.
More information
If you're curious about recipes, mechanics, and more, check out the Github wiki here.