Embedded Systems/Intel Microprocessors
When talking about Intel microprocessors, the first words that come to mind might be "Pentium" or "Celeron", or any of the other high-performance, expensive PC chips that are on the market today. However, Intel maintains a very impressive list of legacy parts that can be adapted for embedded systems. The beauty of using these microprocessors is that they are frequently very cheap, and they will all use the standard x86 assembly language, so that developers can program, assemble, and test from the comfort of a PC.
8086 and 80186
8086 and 80186 processors are available in heavily integrated packages. They are usually available in DIP form, and are relatively cheap (10$ or 15$ range). These processors might not be as good as an 8051 in an embedded environment, but the ease of programming, and the familiarity that many programmers will feel for these chips can more than make up for the cost.
i386 Embedded Processors
The i386 microprocessor is a modified form of the Intel 80386 microprocessor with a few notable differences: an integrated FPU (originally wasn't standard until th 80486), and a variety of different, small form-factors. One of the major benefits of an i386 microprocessor is that it can be programmed easily using most standard C compilers and x86 assembly language. In fact, many times no additional settings need to be changed in the compiler, except maybe to not link to the standard libraries on the host system.
i386 processors are 32 bit processors, and are frequently very economical choices when a 32bit processor is required. Also, i386 processors frequently have very low power consumption, and generate very little heat. Remember, Intel has been working on this architecture and the general design of this chip continuously for many years now.
X-Scale Embedded Processors
The X-Scale processor is an ARM based device, designed for embedded systems requiring high performance with low power consumption, such as PDA's.
Intel does sell embedded varieties of all its chips, from the 486 up to the Pentium 4. Keep in mind, however, that these chips have all the power of their PC cousins, but in a smaller package. Therefore, it can be expected that they will all be considerably more expensive then the desktop chips. Also, with some of the higher performance chips (pentium and up), since the size has been aggressively reduced, and because they have been highly integrated for embedded environments, heat can become an issue (meaning you will need to invest in fans and heat sinks as well).