The history of Twitter's @reply Jul 10 2012
Evan Henshaw digs through some ancient tweets to find the first usage of the now-ubiquitous @reply on Twitter.
The @reply was created on Thanksgiving day, November 23rd, 2006. One wonders if this was the first case of geeks using twitter to avoid their family on thanksgiving.
See also the first use of hashtags on Twitter. It's funny that so many of the things that make Twitter compelling weren't actually invented by Twitter but by the users and developers.
Update: Garrett Murray finds an even earlier usage.