URL schemes are a very useful but simple thing. They are basically just links that allow your website or app to open an app and pass some given parameters on to that app. I collected some of them over the last years, these are the most useful ones:

Mail

Create a new mail with a given recipient

Replace "recipient@example.com" with the email address of the recipient

mailto:recipient@example.com

Create a new mail with a given recipient and subject

Replace "recipient@example.com" with the email address of the recipient and "yoursubject" with the subject that should already be entered for the user. Special characters in the subject should be encoded like in an URL.

mailto:recipient@example.com&subject=yoursubject

Create a new mail with a given recipient, subject and message

Replace "recipient@example.com" with the email address of the recipient, "yoursubject" with the subject and "yourmessage" with the message that should already be entered for the user. Special characters in the subject or message should be encoded like in an URL.

mailto:recipient@example.com&subject=yoursubject&body=yourmessage

Phone

A phone app that supports this standard url scheme needs to be installed.

Start a phone call with a given number

Replace "0123456789" with the phone number you want to be dialed.

tel://0123456789

Text Messaging

A text messaging/SMS app that supports this standard url scheme needs to be installed.

Write a text message to a given number

Replace "0123456789" with the phone number you want the text message sent to.

sms://0123456789

Apple URL Schemes

Here are some additional URL schemes that only work for Apple products:

Call a given FaceTime account

Replace "recipient@example.com" with the the email address or phone number, which is connected to the FaceTime account that should be called.

facetime://recipient@example.com

Write an iMessage to a given account on the Mac

Replace "recipient@example.com" with the the email address or phone number, which is connected to the iMessage account that should be called.
Under iOS you need to use the URL scheme for text messaging instead.

imessage://recipient@example.com

Reedem a given promocode in the iTunes Store

Replace "PROMOCODE" with your promocode. iTunes (Mac & Windows) or the iTunes Store-App (iOS) needs to be installed.