GMAIL

Gmail was one of the first tools that had a significant impact in my e-life.
Before Gmail, I remember backing up and saving pop configuration files and going through a heap of trouble to get my messages synced between my palm pilot and a desktop computer.

Gmail has incessantly increased the tools that are available and has made communication much more efficient, for example through the chat window and the method of archiving messages.
I must admit that the latter was one of the biggest 'climbs' in using gmail. Using labels instead of folders can be quite overwhelming at first, but is quite intuitive after you get the hang of it.

For those reasons, and the integration of all other google products, I wouldn't create an account anywhere else if I ever needed it.

For students, I believe a very important aspect is to make sure they understand this filing system; making sure that messages are not archived if they haven't been dealt with (out of sight, out of mind), or turn them into tasks/add them to the calendar; also making sure that messages are categorized and organized in a way that they are easily retrieved later (perhaps, by creating a label for each subject).

One of the down sides of gmail and its chat window is having students becoming distracted by it when they're supposed to be working on something individually. This, however, can be easily avoided by having a vigilant teacher (hint: students hardly giggle if they're working on a Biology paper).