At Zoho, we provide APIs for a good many of our products and they have all been well received. A sample list of our API partners here. The latest one to join the list is Elementool.
Elementool provides web based project management tools for help desk, bug and defect tracking and time tracking tools. Elementool has implemented Zoho's API in their Docs module. All uploaded documents, spreadsheets and presentations to the Elementool's Docs module can now be viewed using Zoho. Not just viewing, you can edit the file too and when you save the file again, all changes get reflected in your uploaded file in the Elementool account.
Such a functionality is useful if you are uploading a lot of your project/work-related documents, spreadsheets & presentations online and want them viewed or edited then and there in the browser itself (instead of downloading each of them and editing them using a desktop application). Try Elementool out and see if it fits your needs.
We have a couple of other API collaborations happening too which we will blog about in the days to come. Stay tuned.