Robin Hanson recently mentioned "writing therapy" as potentially having surprisingly large benefits. In the example he gives, recently unemployed engineers who write about their experience find jobs more quickly than those that did not.
The meta-analysis paper he links to was pretty lame, but I found another meta-analysis, "Experimental disclosure and its moderators: A meta-analysis", on a somewhat broader topic of Experimental Disclosure that appears to be much better.
My judgment is non-expert, but it looks to me like a very high quality meta-analysis. The authors use a large number of studies (146) and include a large number of potential moderators, discuss their methodology in detail, and address publication bias intelligently.
The authors find small to moderate positive effects on measures of psychological health, physiological health and general life outcomes. They also find a number of interesting moderating factors.