Consultants and outsourcing in development

For those of you in development, how many different consultants are you using to outsource specific tasks, and which of those tasks have you found are best to keep in-house?

I'm thinking using consultants overall will be a bigger net positive for the developer because the consultant's work is going to have start and ends that are aligned with the project. Employing someone in-house to cover that specialized task will end up costing more because it continues a financial obligation from the employer even if there is no work to be done.