The website is not readlater.com but instapaper.com.
You just need to register a username, password is not even necessary. If you need some security, just setup the password later. Drag a "read later" icon to your browser toolbar (it support firefox, not sure for other browser), then it is done. Is simple, right?
The idea of the service is, if you found some good websites for reading, you can bookmark it and read later. After you read the website later, it will be remove from your bookmark by default. You need to disable the default setting from your configuration.
Instapaper is an online bookmark service, but it is different from delicious. Delicious allow you to keep and share your bookmarks online, tag your bookmark, it was build with the community concept. Instapaper do not have the community or network circle (may be they will build one), it even delete the bookmark automatically after you have read it. Instapaper is dead simple, from the registration to the bookmark management process.
For me, instapaper is different from delicous because instapaper have good support on iPhone, which I have eagerly expected from delicious.