If you're a gadget developer, you've probably used the gadgets.* API, a re-namespaced and improved version of the original legacy, or _IG_*, gadgets API. The gadgets.* API has gained wide acceptance, both on Google and non-Google gadget containers, and is the standard API for gadget development.
However, there remains a number of gadgets using the legacy API, primarily gadgets developed for iGoogle, and the time to upgrade those gadgets is now. As of today, the legacy gadgets API is officially deprecated. For a period of one year, gadgets using the legacy API will continue to be supported, and function. After that, the legacy API will be turned off for the majority of Google containers (such as iGoogle, orkut, Gmail, and Calendar).
For more specifics on how the deprecation affects iGoogle developers, and details on coming resources to help in the API transition, check out this post on the iGoogle developer blog.