While we have no official list of all of the IDX vendors we work with, attached is a page from the old CMLS IDX Manual that lists some of the more popular IDX vendors out there.
This list is not exhaustive, and has not been updated in over a year, but it should provide a good place to start if you are looking for an IDX vendor to help you out with your website's IDX needs.
For more information regarding IDX (including rules, forms, vendor payments, etc.) please check out this page from the SmartMLS corporate site. This is a great resource not only for Brokers, but for IDX vendors as well.