Willamette is the codename for the first Pentium 4. In 1998, when the Pentium II was new, Intel started developing the Netburst microarchitecture with 1GHz as target clock frequency. The oldest Pentium 4 I've seen can be found here and, as far as I know, first working samples of the Pentium 4 is the QT69ES.
On 20 November 2000, Intel released the Pentium 4 at 1.4 and 1.5GHz. It was an expensive chip and it needed expensive Rambus (RDRAM) memory in order to operate. Motherboards with the slow but cheap SDR-SDRAM (which caused a performance hit for bandwidth hungry the Pentium 4) came later. The introduction of the SDR-SDRAM capable motherboards caused the Pentium 4's sales to increase a lot. The Pentium 4 Willamette came in versions ranging from 1.3GHz to 2.0GHz. Budget versions, the Celeron, had 128KB L2 cache instead of 256KB L2 cache. Willemette was launched using the S423-socket but was later available for the S478-socket, just like the newer Pentium 4 Northwood.
Willamette did not perform very good, but paired up with RDRAM it's performance was not bad. AMD could offer a much cheaper Athlon (Thunderbird) which performed just as good.
New features in the Willamette were SSE2 instructions and clock-throttling. The latter made the processor run slower in case it started to overheat. In case of a fan-failure the processor won't be damaged.