You're right to be suspicious of most of the websites which offer 'work from home' opportunities. They nearly all require money up front and you never get paid for the work you do.
In practice, the best way to work from home is probably on a self-employed basis, e.g. by offering an ironing service.
However, I do know of one legitimate job offer. It's working for Deya Ltd, who deliver Yellow Pages (and other directories) throughout the UK. They pay workers to phone customers, to ensure that their they've received a directory. (i.e. they're checking up on their delivery staff, to make sure that they've really done the work that they're being paid for). I don't know how well it pays but, as a team leader for Deya's delivery teams, I can assure you that Deya is a legitimate company who ensure that their people working for them always receive at least the National Minimum Wage. You can find details here:
http://www.deya.co.uk/index.aspx?pageref=5130
Chris