As we keep telling you,
Gallup has the latest:
The percentage of Americans identifying as political independents increased in 2011, as is common in a non-election year, although the 40% who did so is the highest Gallup has measured, by one percentage point. More Americans continue to identify as Democrats than as Republicans, 31% to 27%.
These results are based on more than 20,000 interviews conducted in 20 separate Gallup polls in 2011. Gallup has computed annual averages of party identification since 1988, when it began regularly conducting interviews by telephone. The prior high percentage of independents was 39% in 1995 and 2007.
Gallup records from 1951-1988—based on face-to-face interviewing—indicate that the percentage of independents was generally in the low 30% range during those years, suggesting that the proportion of independents in 2011 was the largest in at least 60 years.
Looking at the trend lines since 2008 in particular, it's nothing short of amazing that Republicans can convert Democratic stewardship of a lousy economy into an even worse market share. Every defection from two-party membership makes politics less stable, and more susceptible to outsider candidates and issues that run counter to the unloved status quo. Though Gallup also cautions in the piece that pre-election-year independent numbers typically come down a notch in presidential years.
More analysis of the subject can be found in The Declaration of Independents: How Libertarian Politics Can Fix What's Wrong with America.