I use BibleGateway exclusively for finding Bible passages and linking them to my blog posts. You can also use this site for Bible study and research and use a reading plan to read through the Scriptures.
The Blue Letter Bible is an online reference library with Bible study teaching and commentaries. You could also sign up for a daily Bible reading by following their Twitter account. This could be useful for mobile users to have a daily Bible reading sent automatically to their phone.
On YouVersion, you can use their reading plan or be a part of a Bible Study studying the Bible together with a group. You can also use YouVersion to post Bible verses in either your Facebook or Twitter status. [Also see the YouVersion Twitter account and the YouVersion YouTube acoount]
These overviews are definitely a bit sketchy for what you can potentially do with any of these websites. Please let us know how you use these online Bibles.