Welcome to the forum. Fidelity and Vanguard are probably the hands down favorites for places to stash money. You mentioned brokerage accounts and I'd encourage you to do some research about no load, low fee index mutual funds available at both Fidelity and Vanguard. Research show that a diversified portfolio of low cost mutual funds out performs most individual investors stock portfolios over the long term.
As for your other question, a Roth IRA is the best place to put money after maxing out any match from your employers to your 401k. After maxing out the Roth, max out the 401k and then save "after tax." You should also have about 3 to 6 months of savings available out of any retirement account for emergencies. If you are saving for a car or house, these funds are also outside of any retirement accounts.
The object of life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane -- Marcus Aurelius