What's the easiest way to get fast USB serial communications?
I have used the FT245 chip (now the FT245R) with its bidirectional 8-bit FIFO interface in an FPGA-based project, and it worked quite nicely.

I'm not sure how it interacts with the "standard" OS X and Linux drivers; we used it with the D2XX driver on Windows. As far as the USB host is concerned, it functions just like a serial port, except there's no baud rate generator and no hardware handshaking.