Collaboration tools are nothing new, but they don't always make it easy to find what you're looking for: conversations, files and other resources may sit in entirely different places. Flickr co-founder Stewart Butterfield wants to solve that through Slack, a service that just started taking sign-ups for its private preview. The collaboration app centers on a universal search interface that simplifies locating conversations and shared files, even if those files are hosted by a third-party provider like Google Drive. Both messages and notifications sync across dedicated apps for Android, iOS, OS X and Windows; Slack can also pull in content from outside tools like bug trackers, help desk clients or Twitter. The company is planning for a public launch in the fall, but those who just can't wait can ask for a peek at the source link.