I haven't ever looked to see if there are just platforms to sign up for, as that would seem more like freelancing. Contract houses are companies that manage the contractors. When a company needs some people, they call up the contract house, the contract house then provides some candidates for the company to choose from.

Maybe I'm just old and this space has been disrupted, I see this article from Upwork on it [0]. I would imagine your larger companies will still go through an agency.

I only did one short contract early on in my career, but I've known people who have done it for decades. They like the variety. Some contacts can be short, just a few weeks, while others can be for months or years. Some companies also do contract to hire... start as a contractor, then get hired in as a full time employee.

Which one may depend on your area and what kind of stuff you're looking to do. I can list some I've interacted with in some way over the years, but I can't speak much to how good or bad they are (maybe some others can). Here are a few to give you a jumping off point... TEKsystems, Robert Half, KellyMitchell, Kforce. You may also want to look up "IT Staffing", "Tech recruiting", stuff like that seems to be what they go by now.

[0] https://www.upwork.com/resources/what-is-a-contract-job

Around here, Toptal is usually recommended against similar ones.