Your blog website has an RSS feed and you want to make it a 1-click process for Bloglines readers to subscribe to your feed. Very simple - you would just put that tiny 80x15 pixel wide graphic that says "Add to Bloglines" .

But there are tons of other newsreaders like Google Reader or Newsgator or MyYahoo or MyMSN... it will be impossible to add a RSS subscribe button for each of the newsreader.

Fortunately, there are some "One Button: Multiple Readers" services that allow readers to decide which RSS Reader they want to use. Only a single RSS Feed Subscription button or link is placed on the website reducing clutter caused by those buttons.

There's one more advantage: Website publishers do not have to make any changes to their site when a new RSS reader is released or an existing service changes URL parameters, size of graphic or put a new image altogether.

SoloSub http://solosub.com/

When users click the SoloSub link, they are invited to select which RSS reader they want your RSS feed to be sent to. Solosub supports all the popular online aggregators and desktop readers.
SoloSub is for button addicts

quickSub http://www.methodize.org/quicksub/

quickSub adds a CSS-Javascript function to your webpage - When your roll your mouse over the RSS feed feed button on your web page, quickSub displays a drop-down list of one-click subscription links to the most popular aggregators.
QuickSub 0.3.5 released

SingleSub http://singlesub.sourceforge.net/

Another Single Subscribe button service - Source code available here.
SingleSub v01. released

MultiRSS http://www.multirss.com/

Similar to SoloSub but MultiRSS supports almost all newsreaders. If the newsreader is not present, MultiRSS redirects to the original feed URL. MultiRSS is a service of UrlTrends.

Add To Any http://www.addtoany.com/

AddToAny is new entrant in the game. Let you narrow down the list into feed readers and social bookmark managers to help you quickly locate your favorite newsreader from the big list.
One button for a vistor to subscribe to a feed

If you are aware of other services, do drop a comment and I will add it to this list.

And if you are not happy to outsource your feed-subscription jobs to a third-party, stay tuned. We'll soon write a detailed entry on subscribing to individual news readers.