Does there exist any drop-in USB I/O controller?
What you are describing there is known as a Microcontroller Development Board.  There are many many different makes around.

The most common one is called the Arduino (made in Italy), and consists of an Atmel MCU (different ones are available) on a PCB with a USB interface (usually another Atmel MCU acting as a USB CDC/ACM interface to the main MCU's UART port).

Other makers of similar boards include chipKIT (who use the PIC32 microcontroller), Maple (who use ST Micro ARM chips), as well as the chip manufacturers themselves making them (such as Ti's Launchpad series of boards).

Most of them are programmed in C++ using a semi-standard API (the Arduino API is seen as the standard, and most of the others try and emulate it fairly closely), but some of them do also provide BASIC for programming in as well.

There are "bigger" boards as well, such as the Raspberry Pi, and the Beaglebone, which run a full blown version of Linux (some other operating systems are available as well), and often provide Ethernet (and/or WiFi) interfaces as well as USB ones.

Boards range in price, from just a few dollars (such as the Launchpad, IIRC mine cost $4.50 including delivery) up to $50-$100, such as the Beaglebone.

If you are looking for something less programmable there are modules based around the FT285R chip available.  This is a USB to parallel FIFO chip, to which you send bytes and the equivalent 8-bit value is output on 8 IO pins.  It can also be used as 8 inputs if you wish.  That would mean all the programming would be done on the PC as the chip isn't programmable in any form at all.