Right now, the most famous Fiat (at least in the US) is the Fiat 500. The 500 has become the iconic Fiat, the mascot of Fiat, and while it's certainly a car that earned that status, I've always felt that it's older big brother, the Fiat 600, never really got the credit it deserves. Let's try and fix that.
First, there would be no Fiat 500 without the 600. The Fiat 600 defined the fundamental design of the 500, and it wouldn't be out of line to say the Fiat 500 is really just a scaled-down 600. The 600 was designed by Dante Giacosa, and was the first of a long line of rear-engined Fiats, spawning derivatives that ranged from beach cars to serious sports cars to one of the first minivans.
The basic design was somewhat similar to a Beetle, but with some pretty key differences: water-cooled inline engine, unibody construction, and a good bit smaller. Clever packaging let the tiny car seat four and even include a little 2.3 cubic foot trunk — maybe not original Mini-levels of space utilization, but it wasn't bad at all.
The main sporting version of the Fiat 600, the Abarth version, was actually quite the Mini-fighter in the day, and proved to be a worthy, tiny adversary.
Even if it's the 500 that gets the Pixar characters modeled after it, the 600 was wildly successful back in the day, hitting a million copies afetr only six years on the market. SEAT versions in Spain were so popular that they became a symbol for Spain's economic growth, and SEAT even offered the only 4-door version of the 600, the SEAT 800.
The Fiat 600 was also the car the Soviets chose to copy for their mass-market entry-level car, the Zaporozets 965, though they gave it a V4 and a bunch of other changes.
The Fiat 600 was also the base for the amazing Multipla derivative, which is probably the smallest thing you can cram six people into without being arrested for human trafficking. There was even a full van variant, the 600T. This was one flexible little platform.
Today, Fiat's got a modernized 500 to keep that car's memory alive, but nobody seems to really care about the less-little 600. The 600 was doomed because it wasn't at that tiny extreme filled by the 500, which acted as a sort of Cousin Oliver to the 600: the big wigs brought in something even smaller and cuter, and that doomed the slightly larger, more practical brother to relative obscurity.
So, take a moment today and give a little thought to that not-quite-the-smallest of the Fiats, the noble, hardworking little 600.