On April 12, 1961, Yuri Gagarin became the first human to see firsthand the blackness of space above our home planet's thin atmosphere. Since there's lots of thoughtful reporting and commentary being posted on this anniversary, I thought it'd be more useful to link to some particularly interesting posts than to add in my comments.
A few sites focus on imagery. Space.com has a nice 30-picture photo album of the historic first flight and its aftermath. Today's APOD is selected to show a view similar to what Gagarin would have seen.
Visit Roscosmos for the view of the anniversary from Russia. There are two AP stories by Vladimir Isachenkov, one looking back and one looking forward. Daniel Fischer has a very thorough page full of links to articles across the Web (the blog entry is in German but most of the links go to English articles).