Yappy is described as 'SMS from PC - Send and receive text messages from any browser using your current android phone'. There are seven alternatives to Yappy for a variety of platforms, including Online / Web-based, Android, Windows, Android Tablet and iPhone. The best alternative is Pushbullet, which is free. Other great apps like Yappy are AirDroid (Freemium), AirMore (Free), Mightytext (Freemium) and Free Online Phone Number (Free).
Text/SMS from your PC with a free phone number from Pinger via textfree.us. The easy to use web app gives you a phone number and lets you send text and SMS messages, with the option to pay for minutes to call people as well.