On my other blog, I've been quite surprised by who has asked me to do marketing for them! Often, it's gift companies, thinking I can promote them because my blog is partly aimed at the gift market (although I make them all myself).
Once, I was even approached by Not on the High Street (who of course have a whole marketing department on their payroll!) with articles they expected me to publish on my blog (with links to their site). Flattering, but quite irritating, that they thought little-old-me could help them! Irritating? Yes, because of course they are a direct competitor and there's no way they'd mention me on their blog unless I signed up (with a big fat wad of money changing hands) to open a shop on their site.
Most of the time I just ignore these emails. I've come to realise that if you considered each unsolicited email carefully, you'd never get anything else done.
However, you may be able to negotiate something that benefits both of you, depending on what you're being asked to promote. I can see this might work for some craft bloggers, especially within tutorials.
It isn't just about big companies. The people who are really important in this are the micro businesses - the people who are painting, sewing, woodcutting, crocheting, making, baking and knitting in their little garden studios, garages, sheds, kitchens or spare bedrooms.
These are actually the people I had in mind when I set up Handmade Harbour and I'm more than happy to promote them. Of course, this blog might get promoted in return when it mentions a micro business (although I don't make this a stipulation of being featured here, it is of course welcomed).
This really well-thought-out blog post on Oh My! Handmade is something that you might want to read. It will help you formulate in your mind whether you want to promote other people, craft suppliers, businesses, etc., on your blog and decide whether you want something in return for doing so.