RDRAM - that was a fiasco bordering on conspiracy which was driven by nothing more than Intel's greed and was totally oppposed to consumer demand. The story goes that Intel forged a deal with RAMBUS whereby if Intel could push thier RDRAM standard to the biggest share of the market they would get a 50% stake in the company and various other favourable deals.
The first attempt that this was Intel's i820 chipset for the PIII-E with a 133MHz FSB. This platform was originally designed to work with RDRAM and RDRAM only. Unfortunately for Intel, board makers and consumers were not prepared to purchase RDRAM which at the time was extremely expensive and had very limited availability and appeal. In a rushed last minute gasp to try and save the i820 platform they converted the chipset to PC133, unfortunately they didnt do a very good job and users experienced horrific reliability problems which eventually led to a product recall, refunds for all purchasers and the canning of i820.
Wind forward to the release of the P4 - Intel now had a golden opportunity to monopolise RDRAM for thier own gains, so when the P4 was released the only RAM supported was RDRAM, which was not just expensive but also slow. Theres actually very little about RDRAM which was good, it had a very narrow bus, just 16bits, which meant to match SDRAM (64bit) it had to run 4x faster, but thats just the start. RDRAM was very difficult to manufacture and many manufacturers were reluctant to do so due to low yields and having to pay royalties to RAMBUS for the privelage. RDRAM was never popular with consumers, it was expensive, it was slow, it ran very hot, Intel tried to push it by bundling it with the Pentium 4 but in the end boards came to market for the P4 that supported SDRAM - SDR SDRAM at first too, which although a better and cheaper alternative to RDRAM was hardly state of the art as AMD platforms were already running DDR266 SDRAM. RDRAM faded away and in desperation RAMBUS tried to claim that they invented SDRAM (a complete fabrication) and tried to charge royalties to those who make it. Some of the companies stupidly paid up but others contested it - and won. The directors of RAMBUS have now been tried for fraud and Intel has tried to distance themselves from them ever since.