Online Maths Tools
Wolfram Alpha (solves most problems, you can also ask it to tell you a joke) - http://www.wolframalpha.com/
Desmos (graphing tool) https://www.desmos.com/
Symbolab (algebra, trig and calculus solver that gives worked solutions) https://www.symbolab.com/solver
Geogebra (excellent apps for Geometry and Statistical Distributions) https://www.geogebra.org/
Stat Trek (statistics tools and tables) http://stattrek.com/
Other interesting websites
Famous Curves Index - http://www-history.mcs.st-andrews.ac.uk/Curves/Curves.html
Free customisable graph paper to downlaod http://incompetech.com/graphpaper/
http://statpages.org/index.html - links to statistical software and information (advanced)
LaTeX tutorials - http://www.mrskrummel.com/tutorials.html
Really hard maths competition past papers (Putnam maths competition) http://kskedlaya.org/putnam-archive/
Randomness - http://www.random.org/randomness/
School level Maths Websites
NRICH Maths enrichment materials for all ages, in particular...
NRICH STEP contains some resources on STEP preparation.
CIMT Plymouth - well thought through resources from primary to A-level (don't be fooled by the old fashioned looking website!)
Cambridge STEP advice and resources - information direct from Cambridge - follow the link at the bottom of the page for the document 'Advanced Problems in Core Mathematics', a large book of STEP style questions with hints and solutions.
Oxford MAT papers - for the Oxford entrance exam
http://www.mathshelper.co.uk - GCSE/A-level/Oxbridge/Olympiad resources
http://corbettmaths.com/ - Worksheets and videos as well as interesting maths challenges
http://www.physicsandmathstutor.com/ucas/personal-statement/ Some excellent advice on writing personal statements
http://www.drfrostmaths.com/index.php A really excellent set of slides, worksheets and more - really good resources for students and teachers!
Mini papers on all GCSE topics and A level topics:
Resourceaholic blog http://www.resourceaholic.com/2017/04/gemawards2017.html