The Pilot Hi-Tec-C is legendary amongst pen fans.  Essentially it's a range of extra-fine needle-tip gel rollerball pens.  The reason people love them so much is that not only does 'extra-fine' actually range from merely extra-fine, through ultra-fine and all the way down to hyper-fine, the refills write incredibly smoothly and contain top-notch bio-polymer gel ink.  The most well-known example in the UK is the Pilot G-Tec-C4 (in Europe the G-Tec-C designation tends to be used instead of Hi-Tec-C, but they're the same thing).  G-Tec-C4 is an ultra-fine 0.4mm rollerball writing a 0.2mm line in a choice of 10 ink colours.  At the finest end of the scale is the amazing Hi-Tec-C 0.25, writing an astonishing 0.13mm line.  The popular Coleto multipen introduced not only the custom colour choice to multipens, but used Hi-Tec-C ink into the bargain - a brilliant product.  You can even add Hi-Tec-C goodness into your compact pen or multipen using the D1 refill.