blockchain

Blockchained India

data science

List of Data Science Meetups Worldwide
Silicon Valley Startup: Idea to IPO
Data Driven NYC | 17k+ Members
SF Data Science | 15k+ Members
Applied Singularity | 14k+ Members
Nubank Data Science & Machine Learning | 11k+ Members

mathematics

Association of Teachers of Mathematics

programming languages/javascript

Frontend web developer meetup, Newcastle upon Tyne

programming languages/python

BangPypers
Python Meetups | Worldwide

programming languages/r

List of Some of the Big Meetup Groups Worldwide
New York Open Statistical Programming Meetup | 10k+ Members
Data Science Festival - London | 9k+ Members
Greater Boston useR Group | 3k+ Members
Analytics Vidhya Bangalore | 4k+ members
Taiwan R User Group/MLDM Monday | 6k+ Members
London Data Science Workshop | 4k+ Members

programming languages/ruby

Bangalore Ruby Users Group

software tools/rails

Bangalore Ruby Users Group