When most people hear the phrase "work at home", they immediately think of the infomercials and work at home scams and schemes that you see all the time. You tell someone you work at home, and they think 'Oh, well you must have started your own business.'
However, this isn't always the case. You can find ways to work at home without investing any money into a business opportunity or marketing scheme. There are thousands upon thousands of legitimate jobs out there, and as long as you know where to look you can find the ones that suit you. There isn't a magic trick to finding or applying for these jobs; it's really all a matter of being informed. I can tell you from experience that once you do find these jobs, the application process and waiting game will be similar to that of traditional jobs.
I guess in a way, I do sort of have my own "business." However, it's just me in my office, doing my freelance writing and offering my skills to whoever needs something done. I have some gigs that are steady, such as this blog, for which I am paid on a regular basis. I also have some gigs that are project based, and come and go, depending on the week. It's all a matter of finding what works for you. But yes, you can find places to work at home without investing.
I found most of my projects on freelance writing job boards, through referrals from previous jobs (which might not be applicable if you're just starting out), and on Craig's List. Craig's List is a work at home job searcher's dream. You can click on all the various cities in the world, and search through available job positings, and there are a surprising number of them that allow you to work at home without investing. It is time consuming and tedious, however, because it's like searching through a GIANT virtual newspaper classified section... A matter of trial and error, and finding the ones that are right for you.
There are plenty of job boards out there, depending on your specific niche, where you can find all the relevant listings from CraigsList and other sites all grouped together for you, to save you the legwork of searching through everything. If the sites want you to pay for job information, that's going to be your call. If you're really that desperate and pressed for time, a $20 a month membership might serve you well. However, I didn't need that. I refused to pay for job listings that I could find for free on my own. So I took the time, and I searched (and searched, and searched) for my own ways to work at home without investing.
It's totally your call. Just know that if you take the time to search for legitimate jobs and apply properly, you can have a work at home job in no time, and it won't cost you "Only XX amount of money for training materials". Think of it this way... you wouldn't pay Wal-Mart to hire you, would you? Well, you shouldn't pay a traditional home-basd job, either. And there are plenty out there that would never ask you to.
It's a process, and a tedious one at that, so don't get discouraged too easily. Just invest a little time and effort, and you'll find ways to work at home without investing in no time at all.