This is really only directed toward companies and schools that are using Microsoft Exchange (2003 SP2 or 2007) but others may find it interesting. Most people have heard of RIM's Blackberry devices and services. It was originally introduced as a server based option that required the Blackberry Enterprise Server (BES) which has either a 5 or 10 seat minimum license and isn't cheap. It is now available for single users using phone company provided email addresses as well. When most people talk about real time cell phone based email (and possibly contacts and calendar) this is what they are talking about.
With the release of Exchange Server 2003 SP2 (and 2007) Microsoft added Exchange ActiveSync (EAS) as a free service in the product. This service does the exact same thing as BES only for free and for a wider range of devices. EAS works with most Windows Mobile 5 devices and all Windows Mobile 6 and higher devices. It also works with most Palm Treo and Centro devices, I say most because it is sometimes carrier dependent. Lastly, Apple has announced that this service is coming to the iPhone with the 2.0 update this summer.
Once enabled on the server all that is needed to configure a cell device is to type in the sever URL, username and password. After that it is automatically configured and begins to do real time two way synchronization. There are two potential gotchas that I must warn about as well. First, you will need SSL on your Exchange Server (or firewall depending on setup) and I recommend an official cert rather than in house created for ease of device configuration. Second, make sure you have an unlimited data plan on whatever cell device you do this on or you will get a big surprise when you get your first bill.
The last part of this is that Blackberry itself now has a way for its phones to connect to exchange without needing BES. It is called Blackberry Internet Service (BIS) and is also free. What BIS does is it allows the phone to use Outlook Web Access (OWA) to simulate the same two way synch without the need of additional software. I don't know the full feature set of BIS since I don't have a device to test with but I have seen it used for email with our server. Again with this there was nothing done on the server and on the phone it was only URL, username and password and it is was working in minutes.
I tried to be high level on this post but if you want more details look at the following links, also feel free to ask me any questions and I will do my best to answer them. Hopefully this saves you some time and money when you have someone that wants real time email on a cell phone device.