Mymaths.co.uk is used at The Langton in various guises. Your class teacher may set you homework to complete on it, but it also an excellent revision tool, covering different topics across the different key stages and year groups. To get the school login, please see me in B11 or use the contact me page. 
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Turtle is a great way to get a (really) basic understanding of how programming works at entry level. It brings back fond memories of learning maths at primary school for me as this was pretty much the height of technology in those days!
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Arcademic Skill Builders is a website with a variety of games for students in the lower school. They are pretty basic, but some are quite good for practicing your times tables - you need to know them off by heart up to 12 x 12! 
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Hegarty Maths is a useful website that has lots of videos available for you to watch on various topics, covering many past-paper GCSE questions. 
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BBC Bitesize GCSE has resources for students across multiple subjects, but this link will take you to the maths section of the website. It helps to break the GCSE course up into chunks - well worth a look.  
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This link will take you to a very useful website, but is aimed more at examination practice than the actual development of your mathematics throughout the course. Feel free to use it, but be sure not to plagiarise your work...!
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For those slightly more, er, mature students, this link will take you to BBC Skillswise which has some excellent videos on basic mathematical concepts.  
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+Plus Magazine is a website I've stumbled upon that looks like it has some interesting articles. Have a read and let me know if you find anything really interesting!