After source code of OVM (Open Verification Methodology) which is based on AVM (Advanced Verification Methodology) by Mentor Graphics, now source code of VMM (Verification Methodology Manual) for SystemVerilog by Synopsys , can also be freely downloaded. Synopsys has released its entire VMM library source code under Apache Open Source License. It can be freely downloaded from http://vmmcentral.org/
Not only you can download VMM source code, you can modify it also as per your needs. And if you think, your modifications should become part of VMM library, it is also possible. If VMM designer get convinced about your modification, your modification might become part of VMM source code. Read this Verification Martial Arts: Verification Methodology initiated by author of VMM, for more details about how to convey your modification to VMM designer.