Based off the viaklix module originally developed by ESA Technologies, http://www.esatech.com. VirtualMerchant gateway is used by NOVA Systems / Costco Merchant Services.
Update: a few bug fixes on 12/15/2007.
Jan 3, 2008 Update: a few more bug fixes.
July 21 2008 update: Added in the recommended fixes from the comments below. thanks for the feedback.
Jan 2009 update: Update and beta-testing with Drupal versions 1.6 and 1.7.
Feb 2009 updates