MicMonster is an online text-to-speech solution with natural voices. You are also getting voice style (angry, sad, cheerful) for a few voices. It can transform the text into voice in 48 languages.
Speek lets you record and share voice notes, memos, and responses without an account. Links embed directly to Twitter, making it easy to share voice Tweets, and Speek links can also be shared on forums and embeds on websites and blogs.