The two tools that are widely (and effectively) used for this purpose are JIRA and Confluence.
One of the ways to organize crowdsourcing environment is using the wiki solution Confluence, where people involved can create spaces and arrange in teams for solving certain tasks. Confluence is used by many organizations as it is easy to use and offers quite transparent approach to collaboration. It’s also possible to use something like Confluence Questions specifically designed for crowdsourcing answers.
Another way to organise such type of collaboration is using JIRA instance with a custom plugin on it. Atlassian itself is using it for its charity initiative Makeadiff. It’s also currently planning to integrate it with Confluence in order to take the best features from the both tools.
This was just a very short glimpse on possibilities to use JIRA and Confluence for crowdsourcing purposes. If you’re thinking of introducing such solution in your company, but not sure how to do it – feel free to contact us for a free consultation!