Job Sites
Other Resources
Listed below are the formal internship programs I'm aware of. Probably every museum accepts unpaid interns. If you are searching online, find the web site of the museum you want and look under the "Volunteering" or "Getting Involved" sections. A lot of internship postings also appear on Museum-L.

A Canadian youth internship program that provides work experience for graduates. Internships are 6-12 months and pay $15,000 (cdn) per year. The Federal government has teamed with them to provide Public Sector Opportunities. As a result there are occasionally museum and heritage related positions.

Internship Opportunities at the Smithsonian Institution
Many, many, many internship opportunities in every area of study.

Museum Resource Board
A great resource center which includes a Job Board, Internship Board, and Resume Board. Users can freely post to any board using a form-based interface.

Rising Star Internships
A site featuring internships in various fields from across the US. There are categories for Art History, Museums, and Non-Profit among others.

Young Canada Works
This is a Canadian government work creation program designed to create summer jobs and post-graduate internships for young people (18-30). Some of the programs include working in museums and heritage institutions. See also The National Graduate Registry for more information.