I've created a bulk sms app on force.com How can Distribute the app into the AppExchange? and any people could access and download it for their use
To have an app available on the AppExchange, you'll need to register as an ISV (Independent Software Vendor) through Salesforce's partner program, and have your app pass a security review.
That's about the limit of my knowledge on that particular topic.
The site to go to to start the process is https://partners.salesforce.com