Topsy service has now become the largest searchable collection of past tweets, over 5 billion of them, stretching back to at least May 2008. "Topsy is now the largest searchable index of content posted on Twitter – we recently indexed our 5 billionth tweet and 2.5 billionth link. Unlike most retrieval systems, Topsy organizes its search index in real-time, while still maintaining a long-term history. Our v2 architecture takes our search approach to a new level of scale – it's designed to index over 100 billion status updates and related objects, from any social network."
Beyond being comprehensive, Topsy has a ability to restrict a search using special "operators" or commands — such as "from" — to find tweets from a particular user or the ability to see tweets within a particular date range. Topsy has an advanced search page that makes it easy, as well as a list of commands.