Hello Georg, there exists ADMS project https://github.com/Qucs/ADMS, but now it seems to be stalled and many features of Spectre Verilog-A are missing in ADMS. ADMS allows to compile Verilog-A modules and link it to Qucs or Ngspice/Xyce simulators. Look also at http://www.mos-ak.org/graz_2015/presentations/T_5_Brinson_MOS-AK_Graz_2015.pdf and more publications on using open-source compact modelling tools in my profile.
Dear colleague, The SystemVision Cloud environment has been in operation for about a year. To use it you must register with a username and password. I have personally used this program it is free. Besides all the features of the VHDL-AMS language and features of the SystemVision program from Mentor Graphics are valid and can be used.