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

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!

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