The summer internship/REU is a great opportunity to beef up your graduate student applications or resumes, as well as to learn more physics in more detail.  A research experience of this type will help you figure out which fields most interest you and what type of research you most like to do.  In addition, you will meet a lot of people working in your field, and make friends that will stay in touch for decades.

 We've gathered here a few main sites to look through as you consider which experience would be the most interesting to you.

The National Science Foundation has the most up-to-date information about the highly prestigious REU sites.

Lunar and Planetary Institute Summer Internship Program

A Google search on 'Summer Internship Program +Science' turns up hundreds of opportunities of all kinds.