Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
EZ Texting - EZ Texting provides SMS marketing solutions to businesses of all sizes.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Textmagic - The all-in-one solution for all your business texting needs - marketing campaigns, two-way SMS chats, reminders, notifications, and internal staff communication.
Sipdroid - Free and open-source SIP/VoIP Client
SimpleTexting - SimpleTexting offers SMS and text marketing services for companies of all sizes.