robzilla - 11:56 pm on Feb 21, 2011 (gmt 0)
If these links will be automatically generated (embedded within the widget code), you could give either the reseller or the end-user the option from within the control panel to select one type of link from a list of links with varying anchor text (e.g. "Powered by Wheel's Widgets," "Widget by Wheel," "Thanks to Wheel, Manufacturer of Triangular Widgets"), and the default setting could perhaps be determined randomly for each account.
If the links need to be set up manually (and you'd have to check every now and then to see if they're still there) then, depending on your involvement in the process, I would suggest you determine the 'ideal' anchor text for your link based on the specific relevancy of the niche site or reseller (either of which might lean more towards "blue-red widgets" than "blue-pink widgets," for example, assuming your site offers information on both).
This reminds a bit of the hit counter services of yore; you might have a look at those for inspiration.