I link ... just because.
If I find an external tool that I like and use, I link to it with a normal link and a little blurb about why the link exists. If I *really* like the tool, I write the site owner and let them know and encourage them to keep up the good work.
The link is to help out site users and to help out the other site. I *never* ask for a link back because it would devalue the accolade that I am trying to express to the other site.
Sites wanting to buy links, or begging links get polite answers. But, it is always in the negative because they do not have a place on a b2b site.
BTW, those links, at least in theory, cut support queries. So, its a little bit of selfishness at play :D