usb interface for fpga & Nios
I would recommend either a USB UART chip or a USB FIFO chip.  The USB UART chip will use 2 I/O pins, unless you want to add flow control on top of that.  There are quite a few good USB to serial options out there.  USB FIFO chips require a few more pins, generally 12 pins for 8 data and 4 flow control, but they have some advantages compared to USB UART chips.  First, there is no mucking around with the baud rate.  Second, they can run at a much higher speed - for example, the FT2232 is rated at 8 MBps / 64 Mbps.  The FT245 is a decent solution for a USB FIFO chip, though there may be others.  The FT2232 will also work, but requires a small external EEPROM so it comes up in FIFO mode.  The FT2232 also has the advantage that it can speak JTAG, so you can use a one-chip solution for both communication as well as configuration of the FPGA via JTAG.