What's a good text to speech app?

I'm looking for a text to speech app so I can convert novels I have in text form to audio. I listen to all my books on audio now and about all new novels are in audio form. But I'd like to read some old books that are not available in audio. I know they have excellent programs for this but I'm looking for an independent one that doesn't require internet connection and that I don't have to renew each year. Any suggestions? Hopefully something affordable.