Find below the list of some of the useful websites as UPSC Online Sources for Civil Services Exam (IAS, IPS, IFS, IRS and Others) Preparation (Prelims and Mains) that are categorized into GS Paper I, II, III, and IV:
UPSC Online Sources for General Studies Paper I
UPSC Online Sources for General Studies Paper II
For Constitution, Polity:
UPSC Online Sources for General Studies Paper III:
Other Miscellaneous Official UPSC Online Sources for Civil Services Exam Preparation:
Of Course, there are a lot more websites that can help you with your preparation but these are the major online sources that would enrich your IAS Preparation.
If you find any other important websites for UPSC Online reference and want to add to the list, please leave your suggestions in the below comment section.