CaseComplete has a well-developed integration with HP Quality Center. You can see a video of it in action here: http://casecomplete.zendesk.com/entries/353987-hp-quality-center
Our integration with Smartbear isn't as rich, but still useful. CaseComplete can export all of its data in an Excel/CSV format. Smartbear's tools (like Software Planner) can import from these formats.
Are you interested in using Smartbear's tools? They're certainly closer to us in price point than HP is. Any interest we hear will influence the priority we put on a deeper integration.