Google Voice number at the top of my syllabus with a note for students and parents to "please text me if you have a quick question." From my Google Voice inbox I can reply to text messages and the other person never sees my real cell phone number. Furthermore, it's free for my to send text messages back to anyone who texts via my Google Voice number. Watch the short video below to learn how this works.
This YouTube playlist walks you through many of the other Google Voice features.
On a related note, Wes Fryer recently wrote a post with detailed directions on how to send text messages from a Google Docs Spreadsheet.
Applications for Education
One of the reasons that I have put my Google Voice number on my syllabus this year is because I know that students are much more likely to text a question than they are to email me with a question (nevermind call, that's so 20th Century). The other reason is that I am hoping parents too will prefer the text option over the calling option because I am notoriously slow at checking the archaic voicemail system at my school (I think it still uses actual cassette tapes). If I'm at my computer I can provide almost instant responses to my students' questions.