Information concerning career opportunities for mathematics majors may be found at this web site sponsored by the American Mathematical Society : http://www.ams.org/early-careers/
The general Undergraduate Mathematics Majors page at the AMS provides a wide variety of opportunities for mathematics majors : http://www.ams.org/employment/undergrad.html#summer
Check out The Department of Energy Computational Science Graduate Fellowship that provides outstanding benefits and opportunities to students pursuing a Ph.D. in scientific or engineering disciplines with an emphasis in high-performance computing. See Bernatz for further details or go to the web site http://www.krellinst.org/csgf. Application materials can be downloaded at http://www.krellinst.org/csgf/application. Applications must be received by January 12, 2009.