Some of the best marketing partners you can have are the businesses and services you rely on to help you run your business. They can be a great source of referral business, exposure, and back links for your website.
For example, I've been a fan of aweber.com for a long time because they help me manage my e-mail marketing efforts with excellence. I sent them a nice testimonial several years ago, and to this day they still have it posted on their site along with a link pointing back to my website. I've had thousands of visitors to my site as a result of that one link.
If I ever read a great article online, or a useful blog post, I'm always sure to leave a comment or send an e-mail expressing my appreciation. I also give them permission to use my comments for promotional purposes as they see fit. By doing this I'm helping them out while they help me out.