Already people are coming up with cool uses for this. Digital Inspiration, for example, shows you how to add a Send To link that’ll make a PDF document (via Lifehacker) out of the linked item.

My first thought, though, was to make a Send To link for Instapaper

Instapaper is a simple web app that lets you keep a “reading list” of articles on the web that you want to read but don’t have time for right away. There are already Greasemonkey scripts that tie Instapaper into Google Reader, but a Send To link will work in any browser and shouldn’t break if Google makes changes to Reader down the road.

Update: The day after I wrote this Google added a built-in Instapaper option to Reader. It works great, but a little differently: Whereas the instructions below will instantly add the feed item to Instapaper without any input from you, Google’s built-in method takes you to an Instapaper page where you can edit the Title and Summary of the item before saving it, requiring an additional click–unless you’re not logged in, in which case you must login first and then click the Send To button again. It has the benefit, however, of not making you give your Instapaper username or password to Google. You should choose whichever method suits you best.

So, on to the goods:

How to add a “Send To” link for Instapaper to Google Reader

1. Copy the following URL to your favorite text editor:
   

   https://www.instapaper.com/api/add?url=${url}&title=${title}&username=USERNAME&password=PASSWORD

   Replace the big “USERNAME” with your Instapaper username (or email address, if you use that to login on Instapaper). If you have a password, replace “PASSWORD” with it. If you don’t have a password, just delete everything after your username (“&password” and everything after it).

   Don’t change anything else.

2. Go to the “Send To” tab on the Google Reader Settings page. At the bottom, click on the “Create a custom link” button. A form will appear:

   

   In the “Name” field type “Instapaper” (or whatever you want). Copy the URL you edited from your text editor and paste it in the “URL” field. The “Icon URL” is optional, but use the URL above if you want it to show a pretty Instapaper “I” icon.

   Click on the “Save” button.

3. That’s it!

   Now when you click on “Send To” on a feed item you’ll see Instapaper in the list, and when you click on it your browser will make a request to the Instapaper API. You’ll see a new page load in your browser. If it says “201″ that means it worked and the item was added to your Instapaper reading list.

Troubleshooting

If you see a different number, there was an error. “403″ means there was a problem with your username or password. In that case, make sure that you’ve replaced “USERNAME” and “PASSWORD” in Step 1 with your Instapaper username and password, <strong>not</strong> your Google Reader username and password. Remember that if you don’t use a password when you log in to Instapaper you must remove the “PASSWORD” part of the URL.

A “400″ means there’s something else wrong with the URL. Make sure you copied the whole URL, starting with “http” and ending with “{title},” and that you’ve only changed the “USERNAME” and “PASSWORD” parts.

A “500″ error means something else is wrong and it’s probably not your fault. Try again later.

If you’re curious and technically-inclined you can learn more about the Instapaper API over here.

Update: Based on a suggestion over at Lifehacker I also put together a tutorial for adding an Evernote Send To link to Google Reader.