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My suggetions are mentioned below
https://www.goethe.de/de/spr/ueb.html
http://www.dw.com/de/deutsch-lernen/deutschkurse/s-2068
https://www.deutsch-lernen.com/d_index.php
http://deutschtraining.org/deutschkurs-online/
i would like to suggest you NETFLEX and LINGUEE for learning german language
I would like to recommend udemy.com website. This website has many paid as well as free courses in German and you also get a certification of completion after you successfully finish the course after answering the quizes asked regularly during the course.
https://lp.babbel.com/d/QAE_index.html?bsc=bs_engdeu_us&btp=default&l2=DEU&utm_campaign=BG_NB_SRH_ct_ENGDEU_gEN_cUS_bp&utm_content=ENGDEU_learning+german+free_BP&utm_medium=cpc&utm_source=bing&utm_term=%2Blearning+%2Bgerman+%2Bfree
You could use a great app called DuoLingo for learning many languages, including German. Or, use a web-based learning as well: https://www.duolingo.com/skill/de/Basics-1/1
I have personally not used any German language learning online which is free and I am not aware of your German Language Skill level, so cannot recommend a website however I would like you to consider the following:
www.aufgaben.schubert-verlag.de
www.mein-deutschbuch.de
www.grammatiktraining.de/uebungen.html
www.hueber.de/shared/uebungen/deutschcom
www.deutsch-lernen.com
www.online-lernen.levrai.de
I THINK YOU WILL VERY USEFUL KNOWLEDGE ABOUT GERMAN (DEUTSCHE SPRACHE) BY SEARCHING THESE WEBS
GOETHE INSTITUT, DW GERMAN AND BABBEL.COM
two well-known websites are Duolingo and Babbel