Before everything else...
ENIAC was the first ever fully programmable digital computing device. Its acronym stands for Electronic Numerical Integrator And Computer and it was not really built with scientific goals in mind. The device's main purpose was to calculate artillery firing tables for the U.S. Army.
Now, if you imagine the ENIAC as a less-advanced PC or even as a bulky calculation machine encased in a white case that would take up a large table, well, you would be wrong. It is true that the ENIAC was big, but not as big as a whole table....more like a whole (very large) room. Try to imagine the ENIAC like an organized (that could be an exaggeration as it had lots of wires that where not nicely tied up as in today's PCs) set of (many) wardrobes that sat around a huge room with all kinds of technicians and engineers operating it.
Basically, the ENIAC had no case. If you where forced to describe the ENIAC case the best you could do would be to take some pictures of the room it was held in. The device required nonstop maintenance since it used radio tubes which would fail very often.From huge rooms to actual personal computers
Somewhere before the beginning of the 60's, vacuum tubes were replaced by bipolar junction transistors. This was a major breakthrough as it allowed second generation computers to become smaller and be “stored” in regular cases.
IBM was among the first to produce a truly personal PC that could be advertised and sold to anyone who could afford it (the device was about $3,000, including some extra features). Its name was the IBM 5150 and came with a VDU (a.k.a. monitor), a keyboard and the unit itself.
Every part was encased in a white bulky case and even the keyboard was extremely heavy. There was nothing impressive about its presentation. If you had no idea about what a computer was and what it could do (funny thing is that, back then, computers couldn't actually do a whole lot) you would have probably wondered why someone would use such a small TV and why does it have a typewriter in front of it.
In those times, the evolution of computing power was the main goal of each new computer model. All the cases had to do was keep things together and provide basic ventilation, so that components would function properly. In those days. you could spot a computer from across the room since they all retained a standard shape with no individual differences.
As time passed by, computers became more vertical (literally). In the mid 90's, Intel introduced the ATX (Advanced Technology Extended) form factor. It was designed to replace the old AT form factor and solved some of its annoyances. The fact is that even today, ATX is the most widespread form factor among computer manufacturers (although companies like Dell also build systems using the BTX form factor). ATX is used as a design guide line for motherboard and PC case manufacturers so that people that assemble their own systems or upgrade parts of a system don't come across compatibility problems.
Since 1995, this has been the dominating form factor used in chassis building and remains so today. Around the year 2000, after the ATX form factor had clearly prove to be successful and processing power was obviously not a problem anymore, chassis manufacturers began thinking about how they could make computers more attractive. This was the time for the boring white cases to meet their newborn black sisters. Racism was not a problem here. Although it took some time before the manufacturing of black case systems became widespread, after a while people (most of them computer enthusiasts) embraced cool colored chassis that at some point began taking custom shapes (like the ASUS Vento).
Getting more and more attention....
Chassis manufacturers began using improved materials (like better-processed aluminum and steel) for their products. As more and more enthusiasts started building their own systems, they also began complaining about rough edges. Rough edges were a nasty problem since plugging and unplugging components from the system often required some degree of strength and users would get hand cuts when hitting case edges. As a solution, companies designed most of their chassis with round edges so hitting an edge would no longer cause hand cuts.
Companies like Antec began designing more stylish cases that used piano black surfaces. Black metal surfaces are very sleek and a couple of bright blue LEDs will always make for a proud desktop. At the same time, manufacturers started experimenting with all kinds of mechanisms that would allow for faster and simpler component switching. Drive rails where introduced for optical drives and hard drives. Add-on cards began using all sorts of plastic clips devices that would allow for screw-less installation. The screw-less feature became very popular among some enthusiasts and is still being considered as an important case feature even today.
At some point after the chassis started to evolve, another trend was born, this time concerning the so-called acrylic cases. These were made out entirely of plexiglas and where used in conjunction with LED colored fans. They provided for an impressive view, but the trend didn't last long because dust and finger prints proved to be too much of a problem.
Though the acrylic trend didn't last for long, it gave birth to another concept that is very popular nowadays. The see-through plexiglas side panel became very popular in a very short time. It allowed for a good enough view of the computer components so gamers and enthusiasts could show off at LAN parties. Coupled with a cool neon light and maybe even some UV cables, such systems often won design competitions.
Yes, case modding became a popular occupation among gamers. From custom painting their case to completely reshaping it in order to fit a game theme or just to be popular among the modding community, users where pretty busy coming up with original ideas. Companies like ATI (later acquired by AMD) even hired modding artists to create custom cases for official gaming expos.
Later on, as components were becoming more advanced, heat problems began to surface. Companies rapidly started designing and advertising different airflow techniques. Fan number became a factor of concern in a hot case, convincing manufacturers to include at least 2 (and up to 5 for high-end, big towers) cooling fans in any mid level case.Comfort was being degraded
While chassis manufacturers were solving the heat problem by adding more fans, noise began to be a problem. Peaceful users demanded that companies build silent cases for office use. As so, manufacturers started using soundproofing materials to compensate for noisy fans. Others used high quality, silent fans that were a bit more expensive than regular fans. After water cooling became popular, well-respected companies announced a couple of water-cooling-ready case models. Rubber screws were introduced to reduce hard drive and optic drives noise (also known as vibration attenuation).
For the hardcore overclocking fans, for which none of the above are enough, a little company going by the name of Asetek designed a case using freon cooling technology. Yes, you are right, the same technology used in refrigerators. The VapoChill used a mini refrigerating system to cool down its interior and could allow overclocking fans to test the limits of their components. Later on, Thermaltake designed a similar (improved) model named the XPRESSAR RCS100.
The present day
Today's cases come in all colors, shapes and sizes. Some models even use LCDs to display things like fan speed, CPU temperature and even power consumption. Users can choose between light weight or solid steel cases and have the option to later add a see-through side panel. For picky users, third party companies provide custom laser cutting. So, if you can't find a suitable case out there, just pick the one that most resembles what you want and take it to one of these companies to have it custom modified.