SMS seems to be one of the commonest questions that we see on the forums, so it's well worth a bit of focus. From an enterprise application point of view, one system that has grown a lot in the last couple of years is Wireline SMS, or SMS to Fixed. If you have a PSTN phone in your house that can send and receive SMS messages, then this is using SMS to fixed protocol. It works by sending a little FSK tone burst on the line between ring pulses, so you need some kind of modem decoder in order to talk this language. In the Dialogic product range you can certainly do this with the DMV/A, DMV/B (with GC/R4) and some Diva boards (with the Diva Server SDK). There is some discussion about the DMV here, and about the Diva here.
For high volume SMS, (i.e. usually working partnership with a telco) it is still very common to use SS7 protocols. So to work this way you would have a T1/E1 SS7 link to the telco (or owner of the SMSC), or perhaps an IP link using Sigtran (i.e. SS7 over IP). Our dedicated SS7 products (like the SPCI4 board) and our Sigtran protocol software can provide both of these types of connection. You can see a discussion about the use of the SS7 MAP protocol here.
For completeness I should mention SMPP, which can allow connection via IP, so perhaps some will choose to use Dialogic HMP to access this protocol.
There are also other protocols from times when every SMSC platform had its own protocol, including OIS, UCP/EMI and CIMD2. There are also many legacy access methods: X.25 was used in the past, and also many telcos used dialup bulletin boards (via modem or V.110/V.120 protocol) using a protocol called TAP that was originally invented for sending messages to pagers.
In short, there is far too much choice, and too many questions to ask of telcos/cellcos in order to get started, but SMS-to-fixed and SS7 are still very popular approaches.