I tech reviewed the 2005 Encryption book and at the time, all the EKM integrations were hardware appliances that cost $$. I believe Thales and Safeguard were the two I tested. They worked well, but were expensive, and you needed 2 of them to ensure you could access data.

I believe there are more providers, but I don't think there are any software only solutions. I think all items are hardware based.