Bulk Move is a fairly straight forward plugin which allows you to move all posts from one category to another. This should prove very useful if you are reorganising your categories.
Bulk Delete allows you to delete posts by type and by category. Post types include drafts, revisions, pending posts, scheduled posts and private posts.
The deletion of posts can be restricted to posts older than a set number of days and a first ‘x’ number of posts can be deleted too.
I recommend trying out both plugins if you plan on moving or deleting a large number of posts.