SIAM: Math Matters, Apply It!

https://www.siam.org/careers/matters.php

Try these problems from historical Tripos

A trip into Mathematical Tripos through its history

Looking at this:

http://maths-obsession.blogspot.sg/2015/05/searching-for-mathematical-tripos.html

Many links are outdated, so perhaps here are some updated links:

https://www.maths.cam.ac.uk/undergrad/pastpapers

And these are the pre-1878 era:

https://www.ocf.berkeley.edu/~lekheng/tripos/math-tripos-pre1878.pdf

And lots of others:

https://www.ocf.berkeley.edu/~lekheng/tripos/

https://tartarus.org/gareth/maths/tripos/

https://www.maths.cam.ac.uk/past-examination-papers

https://www.maths.cam.ac.uk/undergradnst/pastpapers

Good Olympics Questions (Computing)

http://wcipeg.com/problems   (from China)

https://noi.ph/past-problems/  (Phillipines)

https://github.com/ranaldmiao/sg_noi_archive (singapore)

http://codeforces.com/problemset

Putnam problems

https://web.archive.org/web/20080124125203/www.kalva.demon.co.uk/putnam.html

Non-linear programming

http://www.math.ku.dk/~moller/undervisning/MASO2010/kuhntucker1950.pdf

http://math.bme.hu/~bog/GlobOpt/Chapter5.pdf

http://orfe.princeton.edu/~rvdb/tex/INFORMSDenver/tutorial3.pdf

http://web.mit.edu/15.053/www/AMP-Chapter-13.pdf

http://www.ai7.uni-bayreuth.de/eolss.pdf

http://www.sce.carleton.ca/faculty/chinneck/po/Chapter16.pdf

http://cepac.cheme.cmu.edu/pasilectures/biegler/BieglerLecture.pdf

Non-linear programming

https://wps.prenhall.com/wps/media/objects/14127/14466190/online_modules/taylor_ims11_module_D.pdf 

https://web.stanford.edu/class/msande310/310trialtext.pdf 

http://www.sce.carleton.ca/faculty/chinneck/po/Chapter18.pdf 

https://wps.prenhall.com/wps/media/objects/14127/14466190/online_modules/taylor_ims11_module_D.pdf