Did you know that Google Talk has its own URL scheme? These "gtalk" URLs can be embedded in web pages or desktop shortcuts to make it easy to message or call someone from Talk. Right now, the two supported URLs are:
gtalk:email@example.com (starts a chat)and
gtalk:firstname.lastname@example.org (starts a call)
Simply replace "email@example.com" with the XMPP ID of the person you want to chat with/call and you're all set.
If you use multiple accounts, you can also add the "from_jid" parameter to the URL to control which account the message is sent from. Here's an example of a URL using from_jid:
As mentioned above, "call" and "chat" are the only URL commands currently supported. If you specify any other string, invoking it will simply cause the Talk client to come to the foreground.
Update: previously I had mentioned that you could use these links to put a "contact me with Gtalk" button on a web page. I goofed; the currently released version of Google Talk won't bring up the invitation UI when these URLs are invoked, so you can't do this quite yet.