As a software vendor, your time is limited. You don't have the time to visit all the software marketing sites out there to find the gems that are useful to your business. These are the software marketing resources that I've found useful for business planning, strategy, marketing and sales in the software services industry.
softwareceo.com - this website provided general information and help related to doing business as a software vendor.
The Business of Software discussion forum - the most active forum I have found for discussion of topics related to doing business as a software vendor. Mainly focused on small software vendors, or micro-ISVs.
The Software Business Executive Report - runs some good articles on the software business, as well as useful conferences.