I've been playing with Phidgets recently, having a lot of fun. They're great for prototyping, but they would be too expensive to use for most of the production purposes I have in mind. I've been thinking that for gadgets I plan to make more than one of, I'd use an FTDI usb-to-serial chip (somewhere around 2.5UKP from their web store, and I think about $5 from the parallax store) and a small AVR microcontroller (for another few dollars). The FTDI web store also sells adapter modules (USB B on one side, header pins on the other) for 10UKP. For the basic make-lights-blink peripheral I have in mind, the FTDI chip alone would suffice, as it's got 5 GPIO pins in addition to the serial port.
I've played with the AnchorChips/Cypress EZUSB before, and it's pretty handy, and you can get them from digikey (page 493 of the digikey catalog lists the full-speed ones at about $10, and the high-speed ones from $15 to $20), but it uses an 8051 core, which is a real drag to program.
So I was pleased to see that Atmel is in the USB game, with their AT90USB1286 and related parts. 128K flash, 8K ram, firmware that lets you program the flash over the USB box, sample code and libraries to do mouse/keyboard/HID stuff (although it doesn't look like the sample code is under a free software license, boo), and a $31 evaluation kit (basically a USB dongle with breakout headers). Digikey has the chips for $14, and a cheaper 64K-flash version is due out soon.
And Atmel also has a handful of ZigBee/805.15.4 chips available, which could be really cool. They include the MAC stack. It's not clear where to buy them or how much they'll cost, though. It looks like there's enough RF goo that you'd want to go with the eval board, and that probably means a couple hundred bucks. But eventually this stuff will make it out to smaller boards.
They're also coming out with a new series of AVRs with really low power consumption, down below a microamp.