Tiburon (
www.tiburon-da.com) has a Spice3f5 interface for Verilog-A.
There's a project (ADMS) on sourceforge that's supposed to connect Verilog-A to ngSpice, an open-source Spice clone, but my last experience with ADMS is that it's not fully mature.
Verilog-A can be run in Spectre, HSpice, Agilent ADS, Mentor Eldo, ...
Verilog-A is a lot nicer (higher-level) language for writing compact models. But if you need it to run in a particular Spice interface, you may need to code directly in C.