As a Chinese teacher, I heard my students telling me which resources they are using and finding useful. I want to organize them and post on italki as an article, so that other learners can find what they need more easily. All the descriptions are written by myself after trying, but there might be something wrong, since I'm not real user. So I post my list here first. If you find anything not proper or anything that I missed, please just tell me!
1.Dictionaries
1) Popup Chinese Dictionary
This is a plug-in of google chrome, which can translate any Chinese words on the webpage to English when you put your mouse over them.
https://chrome.google.com/webstore/detail/zhongwen-a-chinese-englis/kkmlkkjojmombglmlpbpapmhcaljjkde
2)Yellow Bridge
This is an online dictionary which gives you an abundance of relative information of a words or character, such as the HSK level, examples sentences, strokes, related words as well as etymology.
http://www.yellowbridge.com/chinese/dictionary.php
3) Pleco
This is a very popular dictionary on mobile phone for Chinese learners. You can search a character not only by the pronunciation, but also by writing with your fingers on the screen.
2.Exercises for Classic Textbooks
If you are learning some popular Chinese textbooks, such as New Practical Chinese Reader, Integrated Chinese, Huan Ying, Easy Steps to Chinese or others, the following websites might be helpful for you.
1) Quizlet and Memrise
These two websites provide flash cards and exercises such as matching or spelling made by their community. The exercises could help you practice when you are by yourself, but they only focus on the words or phrases.
https://quizlet.com/
http://www.memrise.com/
2) Skritter
This is an app for practicing writing characters which needs you to write on the screen with your fingers. There are lists of characters for different textbooks .
3.Paid Learning Resources
some websites make their own learning materials for Chinese learners. Some resources are free, but most are charged.
1) www.YoyoChinese.com
This website is made by Chinese teacher YangYang Cheng and her team. You will find different series of teaching videos and YangYang hosts the lectures by herself. She speaks good English and could explain every single grammar clearly enough for the learners.
2) www.Chinesepod.com
ChinesePod is making vivid and interesting videos to explains some phrases or language points in Chinese. Also, they provides mobile and online review tools, and live speaking practice with Mandarin Chinese teachers.
3) www.pimsleur.com
Pimsleur is an audio based course that presents phrases in the target language first, and then in your mother tongue for you to translate into that language. The words and phrases are said in isolation or in a suggested context. There is then a pause for you to repeat the phrase, or to recall a previously learned one from memory.
4) www.hackingchinese.com
it is a blog owned by a linguistic who mastered Chinese. You can find articles and resource of learning Chinese on it.

