Seems like a mundane answer, but Wikipedia is an outstanding source without a finnish line. I have created for myself several "Wikipedia books" that consist of aggregated Wikipedia articles on specific quantitative subjects. The results are self-customized world class textbooks-equivalent on any statistical or quantitative domain of your choosing. And, whenever you are intested in a new methodology you can add it to your Wikipedia book and it essentially creates a new chapter for it, including the article. Just learn how to create those Wikipedia pdf files that you turn into a book and you are set.
The above, I think, is a highly underutilized capability of Wikipedia. Although it is really easy to do, I don't know of anyone else that has used this method to boost their learning by creating easily worldclass reference material.
Additionally, depending on what software you use those may have really interesting documentation. Software like SAS, SPSS, Matlab, STATA, XLStat have documentation that represents rich references on the subject. If you are an R user, it also has a ton of reference material on an evergrowing number of quantitative methods. And, it is free (a marked advantage vs. the commercial proprietary software mentioned earlier).