For years now, the "filibuster" has been a mere procedural threat. No Senator has actually held the floor for an extended time to speak against a bill, they've just threatened to do so. But Senator Bernie Sanders is kickin' it old school; right now (5:52 pm EST) he's been on the floor since 10:24 a.m., (with a little relief from his friends) speaking out against the Obama tax "compromise" with the G.O.P. that would screw the middle class and the working poor in order to benefit the richest Americans.
See the video live here.
See also a recent speech by Senator Sanders on the same topic here.