What sort of scale were you looking at? Or, what's the use case? Many end-user apps have SMS capabilities, e.g. Outlook, AOL Instant Messenger, ICQ, Windows Live Messenger, Skype, Yahoo! Messenger.
Otherwise, you could look at a direct-to-SMSC gateway
, which probably could be implemented on DigitalOcean's servers; but would probably require a contract directly with a mobile operator.