Co-Authors Plus makes it possible to assign multiple bylines to posts, pages, and custom post types via a search-as-you-type meta box. Thanks to Mike Patek at Vocativ, version 3.1 includes co-author management via Quick Edit:

Also in this release:

• Updated Spanish translation, courtesy of sergiomajluf.
• Now matches core behavior when displaying author archive on multisite: user of the blog, or previously published author on the blog.
• Breaking change: “Create Profile” link is no longer shown by default on the Manage Users screen. Instead, it can be enabled with the coauthors_show_create_profile_user_link filter.
• Guest authors work properly with Jetpack Open Graph tags. Props hibernation.
• Guest author profile editor now supports a few different fields. Props alpha1.
• New coauthors_count_published_post_types filter for specifying the post type(s) used when calculating the user’s number of published posts.
• Bug fix: Ensure post_author is set to one of the co-authors assigned to a post.
• Bug fix: Filter author feed link for guest authors on the author page. Props hibernation.
• Packages a composer.json file for those using Composer.
• Beginnings of unit test coverage for core features. Increased minimum required WordPress version to 3.7 because WordPress.org unit testing framework doesn’t work reliabilty below that.

Please leave feedback, bug reports, and praise in the WordPress.org forums. You can also get involved with development on Github.