Web: Can't send someone a Direct Message on Twitter but want to share a private link? Privatize lets you post tweets with URLs, but only people mentioned in that tweet can access the URL.
You need to log into Privatize with your Twitter account, add a link and compose a message that mentions as many people as you want. Post it to your timeline, and those people can click the link to see it, after being authenticated by Privatize. If someone else clicks it, Privatize won't let them see the link.
Privatize envisions several scenarios where this might be useful, such as sharing your resume or portfolio, giving out beta codes while generating a buzz, and sharing photos with multiple people instead of separate DMs. The service follows Twitter's recommended best practices for security.