I was able to browse/install plugins all morning and afternoon, but for the past couple hours, I've been receiving "An Unexpected HTTP Error occurred during the API request."
I searched for this error, but the only solved responses said to increase the timeout in wp-includes/class-http.php or wp-admin/includes/plugin-install.php. This just made the error take longer to show up.
Using print_r($response); after line 255 in wp-includes/class-http.php shows this:
WP_Error Object ( [errors] => Array ( [http_request_failed] => Array (  => couldn't connect to host ) ) [error_data] => Array ( ) )
It looks like http://api.wordpress.org/plugins/info/1.0/ is there when I just go to it in my browser... I don't know why this is happening.