Site with collection of exercises in TCS?
100 exercises in the theory of automata and formal languages 

https://algomuse.net/
It's a kind of competition but problems are from research papers. 

Here http://www.openproblemgarden.org/
Some problems have hints. 

https://a3nm.net/work/research/questions/#other-lists-of-open-problems

In this book, each chapter have some exercise: 
https://files.boazbarak.org/introtcs/lnotes_book.pdf