Intel 486 (1989 - 1994)
The 80486DX was released in 1989. It was a 32-bit processor containing 1.2 million transistors. It had the same memory capacity as the 386 (both were 32-bit) but offered twice the speed at 26.9 million instructions per second (MIPS) at 33 MHz. There are some improvements here, though, beyond just speed. The 486 was the first to have an integrated floating point unit (FPU) to replace the normally separate math coprocessor (not all flavors of the 486 had this, though). It also contained an integrated 8 KB on-die cache. This increases speed by using the instruction pipelining to predict the next instructions and then storing them in the cache. Then, when the processor needs that data, it pulls it out of the cache rather than using the necessary overhead to access the external memory. Also, the 486 came in 5 volt and 3 volt versions, allowing flexibility for desktops and laptops.
The 486 chip was the first processor from Intel that was designed to be upgradeable. Previous processors were not designed this way, so when the processor became obsolete, the entire motherboard needed to be replaced. With the 486, the same CPU socket could accommodate several different flavors of the 486. Initial 486 offerings were designed to be able to be upgraded using "OverDrive" technology. This means you can insert a chip with a faster internal clock into the existing system. Not all 486 systems could use OverDrive, since it takes a certain type of motherboard to support it.
The first member of the 486 family was the i486DX, but in 1991 they released the 486SX and 486DX/50. Both chips were basically the same, except that the 486SX version had the math coprocessor disabled (yes, it was there, just turned off). The 486SX was, of course, slower than its DX cousin, but the resulting reduced cost and power lent itself to faster sales and movement into the laptop market. The 486DX/50 was simply a 50MHz version of the original 486. The DX could not support future OverDrives while the SX processor could.
In 1992, Intel released the next wave of 486's making use of OverDrive technology. The first models were the i486DX2/50 and i486DX2/66. The extra "2" in the names indicate that the normal clock speed of the processor is being effectively doubled using OverDrive, so the 486DX2/50 is a 25MHz chip being doubled to 50MHz. The slower base speed allowed the chip to work with existing motherboard designs, but allowed the chip internally to operate at the increased speed, thereby increasing performance.
Also in 1992, Intel put out the 486SL. It was virtually identical to vintage 486 processors, but it contained 1.4 million transistors. The extra innards were used by its internal power management circuitry, optimizing it for mobile use. From there, Intel released various 486 flavors, mixing SL's with SX's and DX's at a variety of clock speeds. By 1994, they were rounding out their continued development of the 486 family with the DX4 Overdrive processors. While you might think these were 4X clock quadruplers, they were actually 3X triplers, allowing a 33 MHz processor to operate internally at 100 MHz.
AM486DX Series (1994 - 1995)
Intel was not the only manufacturer playing in the sandbox at the time. AMD put out its AM486 series in answer to Intel's counterpart. AMD released the chip in AM486DX4/75, AM486DX4/100, and AM486DX4/120 versions. It contained on-board cache, power management features, 3-volt operation and SMM mode. This made the chip fitting for mobiles in addition to desktops. The chip found its way into many 486-compatibles.
AMD AM5x86 (1995)
This is the chip that put AMD onto the map as official Intel competition. While I am mentioning it here on the 486 page of the history lesson, it was actually AMD's competitive response to Intel's Pentium-class processor. Users of the Intel 486 processor, in order to get Pentium-class performance, had to make use of an expensive OverDrive processor or ditch their motherboard in favor of a true Pentium board. AMD saw an opening here, and the AM5x86 was designed to offer Pentium-class performance while operating on a standard 486 motherboard.. They did this by designing the 5x86 to run at 133MHz by clock-quadrupling a 33 MHz chip. This 33 MHz bus allowed it to work on 486 boards. This speed also allowed it to support the 33 MHz PCI bus. The chip also had 16 KB on-die cache. All of this together, and the 5x86 performed better than a Pentium-75. The chip became the de facto upgrade for 486 users who did not want to ditch their 486-based PCs yet.