Here's a list of summer camps and classes that involve learning or using the German language! We have updated a few listings as we have received more information: 

AATG (American Association of Teachers of German)
Age: High School Students
The AATG offers a German Summer Study Program for High School students in Germany. You can find detailed information on this 3-week program as well as other German summer camp information here.
 
GLSC (German Language School Conference)
Age: 12-16
The Program for 12 through 16-year olds is a home-stay program with families in Ingelheim near Frankfurt, Germany.  The Program offers participants an opportunity to use German in a German-speaking environment. The students live with carefully screened host families and participate fully in their lives. For more information, check out this Web site.  Hurry, as the deadline has passed, but late applications will be considered.
 
Carl Duisberg Centers
Age: different age groups
The Carl Duisberg Centers offer several German language course in Germany for students of different age groups throughout Germany. Check out their various summer programs.
 
The Humboldt Institut
Age: 8 to adult
The Humboldt-Institut, a non-profit organization first established in 1977, is one of Germany's premier language institutes. It has several centers throughout Germany and offers courses for children age 8 to adult. For more information, visit their Web site.
 

Goethe-Institut Boston
Age: 6-9 and 9-12
Courses are built for total beginners as well as for advanced learners. June 29-July 10 Monday through Friday 9.45-12.30. For more information, click here.
 
Age: 12-14 and 15-17
For beginners or advanced learners. June 29-July 10 Monday through Thursday 9.45-12.30. For more information, click here.

For general information, contact the Goethe-Institut Boston directly.   
 
Minuteman in Lexington, MA: Leap into German: Eins, zwei, drei!
Age: entering grades K+1
Our own K teacher Silvia James is offering her popular summer course again for children entering grades K + 1: "Learn a language through art, music and play. This unique program is designed for children with no previous knowledge as well as children who already speak some German". July 6 through July 31 from 8.30AM to 11.30AM. Click here for the Minuteman Regional High School Summer Program and go to page 20 for more information on this class.
 
Musikgarten Boston
Age: preschool through 8 years
Musikgarten, the music school that offers early childhood music and movement classes in German, is offering summer classes August 10 through 31st every Monday and Wednesday morning. Click for the summer schedule or contact the music teacher, Dunja Pechstein, directly to inquire about programs. Classes are held in Belmont.
 
"Sommerschulen" 
Age: college
Deutsche Sommerschule am Atlantik
University of Rhode Island
Sprachkurse und Fortbildung
http://www.uri.edu/artsci/ml/german/summerschool/index.html

Deutsche Sommerschule Middlebury
Middlebury College Summer Language School, Vermont
Sprachkurse und Fortbildung
http://www.middlebury.edu/academics/grad_lang/german/academic/courses_vermont/default.htm