Contained herein are attenuator values for both PI and TEE types. The need for standard values can be met by using one or the other
http://www.qsl.net/we6w/text/atten.html
homebrew attenuation pads, describes Pi style attenuator pads for 1 dB, 2 dB, 4 dB and 8 dB attenuation, also step attenuator circuit, also includes Pi and T Network Resistive Attenuation Calculator
http://www.qsl.net/n9zia/wireless/appendixF.html#4
Fixed attenuators can be designed to have either equal or unequal impedances and to provide any amount of attenuation (theoretically) equal to or greater than the configuration's minimum attenuation - depending on the ratio of Z1/Z2.
http://www.rfcafe.com/references/electrical/attenuators.htm
This attenuator is designed for 50 ohms impedance and provides switches for 20, 16, 8, 4, 2 and 1 dB attenuation.
http://www.qsl.net/n9zia/wireless/pics/step_att.png
http://www.minicircuits.com/appnote/eatt4-3.pdf