The addition of a second of time has caused various websites to experience outages as Java based applications are not coping with the additional time.
Google says that it is going to add the additional second over a more extended period, a couple of milliseconds each hour, instead of the whole second at one time.
Linux servers running Debian Squeeze were also reportedly falling over, a quick fix is to disable NTP while you update your kernel or applications. Servers running Hadoop and other Java applications are at risk.
Popular websites that went down include Reddit, Foursquare and StumbleUpon and Mozilla has released a bugfix for mySQL here: http://blog.mozilla.org/it/2012/06/30/mysql-and-the-leap-second-high-cpu-and-the-fix