Originally posted by Eldredge
Interactive Brokers does this as far as I can tell. I'm not familiar with their under $25,000 accounts, but I know if you have over 25,000 you get 4x buying power day and night, and buying power is updated every minute. Margin calls are executed automatically by their software if you exceed your limit for more than one minute - the program will close 100 shares of one of your positions at market (I don't know how it decides which one).
