I just read a great post on "From Toy to Tool", a blog about using cell phones as educational tools, about Google Calendar and text messages. I had completely forgotten about this ability because I have a smart phone (Palm Centro). But, as Liz points out, not everyone can access the web on their phone. Pretty much everyone has the ability to send and receive text messages. Her blog is a great resource for those looking to use cell phones in class, or even just show their students how they can use their cell phones for educational purposes outside of school.
Google has great applications, which are free, for students and teachers to use. I have talked about Google's applications before, because I use them a lot and find them easy to use and very useful.
Google Calendar is a great tool to keep yourself organized. A nice feature is that you can allow others to have read access to you calendar, and you can even combine calendars from multiple people or groups.
If you don't have web access with your phone, you can check your calendar, or even add a new event to your calendar, using text messaging. Here is the Google page with instructions. You do need a Google account and need to register your cell phone when you set it up.
Check out the rest of the applications Google offers HERE.