Johnvey Hwang has created del.icio.us direc.tor, a rich-web interface to del.icio.us that is a great demonstration of client-side web service brokering using javascript. It works in IE and Firefox. Instead of taking the greasemonkey approach, Johnvey's service is launched via bookmarklet.

Other brokering services like the Google Maps hack are not entirely self-contained and require the broker host to proxy information between the main server and the client, thus doubling the amount of network traffic and degrading the overall performance. direc.tor eliminates the need for the broker host to proxy requests by instructing the client to directly communicate with the main server. This approach is very similar to the way Greasemonkey scripts are loaded, except that it is largely platform independent and does not require additional client-side extensions like Greasemonkey. However, the major pitfall to this approach is that users are required to manually create the bookmarklet. Source: del.icio.us direc.tor

Jonvey also posted a how-to on executing foreign-hosted code with bookmarklets in Creating A Client-Side Web Service Broker.