Internships, jobs, summer programs:
AMS national listing of REU's (Research Experience for Undergraduates).
AMS Internships page
AMS Undergraduate page
AMS Bachelor's degree careers page
AMS jobs classified ads (activates March 15, 2008)
Summer Mathematics Program for Women at Carleton
Summer Mathematics Program for Women at George Washington University
D. W. Simpson & Co., actuarial recruiters.
PRYOR Executive Search actuarial positions.
The Mitchell Group, actuarial job search.
S.C. International, actuarial job recruiter.

Professional organizations:
Mathematical Association of America
American Mathematical Association
American Statistical Association
Society for Industrial and Applied Mathematics

Internet resources:
American Mathematical Society undergraduate web page
Mathematical Association of America Math Gateway MathWorld
The Math Forum
The Geometry Junkyard
Geometry in Action
Recreational Mathematics
Penn State's Math Guide
World of Mathematics

Mathematical software:
Netlib collection of math software
GAMS Guide to Available Math Software
Numerical Recipes on-line book

Free online math books:
George Cain, Online Mathematics Textbooks, http://www.math.gatech.edu/~cain/textbooks/onlinebooks.html
Free Online Mathematics Books, http://www.pspxworld.com/book/mathematics/
Alex Stefanov, Textbooks in Mathematics, http://users.ictp.it/~stefanov/mylist.html