You can email articles to your Instapaper account using the secret email address for your account found at www.instapaper.com/save/email.
After that, send a URL as the body of an email to that address to have the article parsed and saved into your account. If you subscribe to mailing lists, you may not be able to cancel later, and you may end up receiving spam in your Instapaper account. You can't change your email-in address, so it's important to make sure it's never compromised.
You can also send text to be stored to Instapaper via that email address, but please note that since it doesn't have a URL behind it, it may not behave the same way as other saved content for things like sharing/annotation.
Tip: add your send-to email to your address book so you don't need to memorize it.