Some of the CAD software come with build in rendering tool. SolidWorks is pretty famous for its build-in rendering tool as the result produced is considered very good with wide selection of material library.
However, there are some software that is designed to be dedicated only for rendering. Recently I tried one of them. The software name is Keyshot.
Compatible for both Windows 32-bit and 62-bit, this software allow you to have 14 days of trial. Download the trial here if you're interested.
For first time user, this software is relatively easy to use. The material library provided is considered pretty wide and it allow you to "custom" made your own material. Assigning material and appearance can be done as easy as using drag-and drop thingy.
Even though Keyshot is an independent software company and is not tied in with any CAD software system, it is capable to import most of the popular CAD files. Assembly file can also be imported directly to Keyshot without losing its relationship. Just click the import button, and you will see your assembly almost instantly.
Despite all the good things about it, by not linked to any CAD software means that the relationship defined in the assembly is also not important. This can become pretty troublesome especially when you need to do rendering of two identical assembly with different instances as you can not modify your assembly in Keyshot. You would need to import it two times and assign the same material twice. That's double the work.
In conclusion, this software has pro and cons. If you need to do rendering on the same assembly with different arrangement/instances, this software might not be suitable for you. However, overall this software can produce decently presentable result and is very user friendly and easy to use.
Some renderings that I've done using Keyshot:
