This is one of our favorite sites. It sets a gold standard in tools, and content, and furthering a vision of "seeking alpha" - the intellectual pursuit of alpha and engaging in lively exchanges, as well as consumption of information to an engaged and sophisticated (mostly) audience.
This is an execution platform. Many people have those associated with their banks, like Citibank, or Schwab, Fidelity, TDAmeritrade, etc... but robinhood offers a free commission / no transaction fees, so is a decent way with a low bar to entry.
This is a slightly more sophisticated site and platform and suite of tools. They have the ability to code in Python and use an extensive database (and ever growing with their recent partnership with Factset [FDS] Open Data project).