Lpc2000

http://www.segger.com/usb_msd.html
http://www.segger.com/usb_bulk_stack.html
http://www.micrium.com/contents/products/USB/USB.html
The last two are the same I believe.

Or brew your own referencing one of the many LPC214x examples:

IAR:
IAR EWARM Kickstart install directory
\Program Files\IAR Systems\Embedded Workbench 4.0 
Kickstart\arm\src\examples\Philips\LPC214x\IAR-LPC-214X\USBMouse

Keil:
http://www.keil.com/download/docs/306.asp  USB HID
http://www.keil.com/download/docs/307.asp  USB Mass Storage
http://www.keil.com/download/docs/308.asp  USB Audio Device

In the 3.2.2 release of FreeRTOS there is a USB CDC implementation:
\FreeRTOS\FreeRTOSV3.2.2\FreeRTOS\Demo\lwIP_Demo_Rowley_ARM7\USB
http://www.freertos.org/

www.at91.com offers a complete Mass Storage example supporting FAT12/16 for 
the AT91 parts.  I haven't examined it too close, but it may utilize DMA.


Regards,
Joel