It have become now the De Facto standard for getting any code published as open source, also for developers it is the first place to look for helper libraries.
Saying that, there are thousands of repos containing great code on github, so how you can find the best ones suitable for your needs ?
Here is the procedure I follow :
1 – Search for proper keyword
for example here I am looking for good animation library to use in ios project
and then filter by the programming language
2 – Sorting results
Try first to sort results by stars then last updated
3 – Look at the results and compare them and see what satisfy your needs
try to read the description well and see how well it is documented.
4 – After picking one look at its issues to see if there is something fatal in it
5 – Look also at its forks to see if it contains any one with improvement you find helpful
6 – Now download the code and try it to see how it will work with you.
A big problem for projects which have alot of forks in github that the internal graph don’t show any of them, which is very annoying, so I found this site which gives you the top forks of a repo, but it is not always working accurately, you can check it here
As outsourcedguru suggested in the comments one other criteria to consider is code size and number of dependency it requires (the dependency issue raised lately on NPM, see this url for more explanation)