The new WordBoard Keyboard app for iPhone is a kind of alternative to TextExpander. Essentially, it's a custom keyboard made of custom text snippet "keys" rather than a typical QWERTY layout. Really, the best way to understand is to watch their promo video:

The base app is free, and allows you to save up to six snippet keys. The $2 "Pro" in-app purchase gives you:

• Unlimited snippet keys
• Ability to import recent hashtags from Twitter and Instagram (great if you use the same ones a lot)
• Ability to import email addresses, mailing addresses, and phone numbers from your Contacts list