Since I frequently use TinyURLs, I’ve come up with a bookmarklet to simplify the process of providing both the TinyURL and the original URL^[1]:

This bookmarklet works in two steps: On the first click, the TinyURL is created. The second click then extracts both URLs from the page and presents them in a newly-created input field, ready to copy and paste.
The original plan was to circumvent the first step by using a hidden IFRAME element. However, due to XSS restrictions, that seems not to be possible.

Limitations

TinyURL currently strips in-page anchors from the original URL (e.g. http://domain.tld/index.html#section becomes http://domain.tld/index.html).
I’m afraid there is nothing I can do about this.

Also, some error handling is currently missing, which might lead to JavaScript errors in case TinyURL change their HTML structure.

Source Code
(latest version available on GitHub)

function extractTinyURL() {
    var txt = document.body.innerHTML;
	var RE = /(http:\/\/tinyurl.com\/\w+)/;
	var original = document.getElementsByName("url")[0].value;
	var tiny = txt.match(RE)[1];
	return tiny + " (" + original + ")";
}

function displayTinyURL() {
	var txt = extractTinyURL();
	var c = document.createElement("div");
	c.style.position = "absolute";
	c.style.top = "25%";
	c.style.left = "25%";
	c.style.width = "50%";
	c.style.width = "border: 2px solid #AAA";
	c.style.padding = "50px";
	c.style.backgroundColor = "#EEE";
	var e = document.createElement("input");
	e.setAttribute("type", "text");
	e.setAttribute("value", txt);
	e.style.width = "100%";
	c.appendChild(e);
	document.body.appendChild(c);
	e.select();
}

if(document.location.toString().indexOf("tinyurl.com") == -1)
	document.location.href = "http://tinyurl.com/create.php?url=" + location.href;
else
	displayTinyURL();

Feel free to suggest improvements!

2008-11-21: added link to GitHub repository

1. see here for my reasoning on this [↩]
2. created using Bookmarklet Builder [↩]