Apart from the books, some imp sources, which can be handy in IIT JEE, AIEEE and other engineering exams preparation are available online. Although we dont advise you to rely on them for your preparation, but at times refering to them may prove quite benefitial and time saving. The best thing about them i that they are available free of cost.Here is a short but effective list of such sources.
Overall
http://www.school-for-champions.com/science.htm_________________
This website provides basic education of each topic related to IIT JEE paper. Though it does not covers all the sub topics for JEE exam but then though this will provide a explain the topic at basic level.
Physics
http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html__________________________
It gives a general idea of topics covered in JEE exam and explain topics, covered in JEE Paper, in brief which consists of definations and formulas. These may act as your short notes which has for last time revision.
http://www.play-hookey.com/optics/______________________________
Here you can study optics in detail. It covers nature of light, reflection and refraction, fibre optics and concepts of Rainbow.
http://www.play-hookey.com/ac_theory/______________________________
It covers the AC circuits and capacitance and inductors in details which is upto a JEE level.
It also covers series and parallel LC circuits
http://www.allaboutcircuits.com/_________________________________________
This website explains each and every part of cuircuit in detail with the help of diagrams and examples. It is further divded into sub topics of the chapter.
Chemistry
http://hyperphysics.phy-astr.gsu.edu/HBASE/CHEMICAL/chemcon.html__________
It is divided into Inorganic, Organic and Physical Chemistry which gives the basic idea of the each topics covered in JEE paper.It is further divided in different topics of the JEE exam which may help you in understanding the basic concepts of every topic.
http://www.webelements.com/_______________________________________________
This provides with all the information of the periodic table. Every element is explained in detail with their pictures, uses or experiments. It explains the properties of each element and explaination for it. This will help you in learning the properties of every elemnt with accuracy.
http://www.chalkbored.com/lessons/chemistry-11.htm
Contains pdf files, short notes, puzzeles, presentation for each topic.
As we all know it becomes difficult to read the whole book of NCERT, which may sometime bore you so this site will make it easy and will also teach you with fun. This website contains pdf files, short notes, puzzles, presentations for each topic covered in cbse sllyabus at 11th and 12th level. This documents may help you in understanding the topic with some fun of presentations and puzzles.
Mathematics
http://hyperphysics.phy-astr.gsu.edu/HBASE/hmat.html__________Gives you a gernal idea of the topics and some basic concepts.
This provides a gerenal information of every topic of the JEE Exam, but it does not contains any detailed information for the topic. This will is only for consulting your basic rules of mathematics as it contains some software which solves your general problems by just inputing the initial values. For eg: In a triangle you are given with 2 sides and 1 angle containg the two sides, and if your task is to calculate third side and other anlges, so if you would be doing this questions then you will be using consine rule and sine rule, this site will give you these type of answers just by inputing the values. It can be used for just checking your calculations at low level. We suggest you not to make a habit of using these softwares, they may ruin your calculation skill.
http://archives.math.utk.edu/visual.calculus/_________________Explains both differential and integral calculus.
All the basic concepts of the Integral and Differential Calulus are given in here which are explained at a very basic level which helps in understanding the Calculus from the root level.