I've found Twilio and Tropo so far as services to aid SMS and Voice apps.
Are there others?
I am interested mostly in real-time speech recognition (i.e. the user can speak things and the system can respond based on that).
Also, if anyone knows what the speech recognition quality is like, it'd help!
For speech recognition and an API-based telephony service, Tropo's about the only option. If you drop the speech reco requirement CloudVox (hosted asterisk apps) could be a possibility. Or you could install Asterisk yourself and use something like Adhearsion to develop your apps.
If you want to expand your scope beyond cloud telephony APIs, then you could take a look at VoiceXML, a W3C standard for building telephony apps. VoiceXML hosts generally support speech recognition. A search for "VoiceXML Hosting" will turn up several thousand options.
In the interest of disclosure, I'm one of the guys behind Tropo and we're a product from Voxeo, the largest VoiceXML host.