How can I make a slightly transparent white Transparent material in cycles, when making the Transparent shader white makes it completely transparent and invisible?
Mixing with another shader:
Using this node setup, the Mix factor is now your transparency slider, and the diffuse color is the color of the material. (in this case, white)
Note that using a diffuse shader as the other shader shades the material slightly. If you want a solid color, you can use an Emission shader but only when seen directly from the camera.
To do this, you can mix it with a transparent shader with the
IsCameraRay output in the Light Path node (Add node > Input > Light path node) as the mix factor.
Seen here with a .3 mix factor: