Definitely, as Marina says. The standard is to list the company for which you actually work. You can add a bullet point with the names of some of the companies for which you’ve contracted if they have some “resume radiance”. You could do something like:
Employer Name Date Started to Date Ended (or Present)
* list first career competency/action perfomed
* list second ” ”
* and so on for each skill/experience you want to highlight
* performed above listed contracted services for a variety of [you could put in a descriptive word or phrase here if you wanted to emphasize the size, importance or whatever of the companies] clients, including [list most “impressive” companies for which you’ve contracted here.
Or, another good approach is to, for EVERY company for which you’ve worked, before you list out bullet points of your accomplishments/duties, you can put a descriptive sentence describing the company. Such as:
Company Name Dates worked
A [size of company in terms of employees and/or revenue] contracted services firm, specializing in [list the types of contract work your employer specializes in].
Then list your bullet points.