Using RadioShack relocatable power tap (120V/60Hz) in 230V/50Hz country
What you have pictured is a "socket strip". It has six sockets connected to the mains plug via a switch. It may also have an indicator light.

It neither converts voltage or frequency. The wires connect straight through.


  Will it cause any problems if I just plug it in the 230V/50Hz outlet to power 230V/50Hz devices? 


It is not designed for 230 V.


  What output frequency and voltage will I get? 


Whatever you connect it to.


  Also, how to calculate max. load/wattage for it, considering it will be used with 230V, and not with 125V, will it be lower?


Why would you even want to do this? No 230 V plugs will fit into the sockets.




  I need such thing, and I have it lying around, but it is made for 120V. 


That probably means the insulation between wires and pins on the sockets is only rated for 120 V.


  I have some 100-240V devices with proper plugs, and I have proper plug adapter — it would be enough if it worked correctly with 230V/50Hz. So you are saying that I can not use it with 230V/50Hz?


Apart from the problems above there is the danger that someone will plug a 110 V only device into your plugboard and destroy the device.

The correct solution is:


Calculate the maximum W (watts) or VA (volt-amps) of all the devices you will have on simultaneously. (There is a technical difference between watts and volt-amps that you don't need to concern yourself with.)
Purchase a 230 / 115 V transformer with a VA rating that meets or exceeds the number calculated above.