USB

USB to TTL

These are basically dedicated microprocessors with the USB stack built in.

FTDI -- See http://saelig.com/ they are about $5

Silicon Labs CP2102 - only available in a surface mount package This is the one I have used; it comes on the breakout board I got for my Lassen SQ GPS.

Another option is to roll your own using a Microchip PIC processor. They have about a dozen chips with USB support. I have not really investigated this option yet. Seems like a lot more work but if you are already building an embedded controller it would eliminate the separate USB chip.

A USB PIC is around $2.00 so it's cheaper.