The teaching of mathematics at the K-12 level is a high-demand field and the need is expected to grow in the future. The place to go for explicit career information is the National Council of Teachers of Mathematics homepage (http://www.nctm.org/).
Actuarial science takes mathematics and statistics and applies them to finance and insurance. Actuarial science includes a number of interrelating disciplines, including probability and statistics, finance, and economics. Check out http://www.beanactuary.org/.
The proliferation of statistics in everything ranging from business to government has induced many organizations to seek math majors. Statisticians use surveys -- for example, opinion polls -- to predict the patterns of behavior of large groups based on relatively small samples. They ask questions such as: How can we be sure that what we predict from our small sample is true of the population being sampled? Probability theory provides the theoretical foundation for statistics. For more information, visit American Statistical Association (http://www.amstat.org)
Computer science is the study of the theoretical foundations of information and computation and their implementation and application in computer systems. Mathematicians, with their training in logical and precise thinking, are highly prized in this field. See the student section (http://www.acm.org/membership/panel?pageIndex=2) of the Association for Computing Machinery for career advice.
Operations research is an interdisciplinary branch of mathematics which uses mathematical methods to arrive at optimal decisions to problems in maximizing or minimizing things like costs or profits. The eventual intention behind using Operations Research is to elicit a best possible solution to a problem mathematically, which improves or optimizes the performance of the system. The group INFORMS (http://www.informs.org/) is the world's largest society devoted to operations research/management science.
Mathematical biology or biomathematics is an interdisciplinary field of study. It models natural and biological processes using mathematical techniques and tools. Results have been applied to areas such as cellular neurobiology, epidemic modeling, and population genetics. The education page (http://www.smb.org/education/index.shtml) of the Society for Mathematical Biology links to schools offering biomath degrees along with a description of the coursework needed.
Cryptography is the practice and study of hiding information. Cryptography is considered to be a branch of both mathematics and computer science. Not just for spies anymore, cryptography applications include the security of ATM cards and computer passwords.
Finance is a field that studies and addresses the ways in which individuals, businesses, and organizations raise, allocate, and use monetary resources over time, taking into account the risks entailed in their projects. Mathematicians can build models to help explain and predict the behavior of financial markets. Several schools offer Master's degrees in Financial Mathematics. A quick web search will take you to their web pages.
Further information about careers in mathematics can be found at: