Which email program you use depends a lot on your type of connection, type of computer, and personal preferences. Here are a few of the main ones:
This is kind of the mother of all mail programs. It is built into most unix systems. Since unix is the foundation of many ISP systems, it is the program you are almost guaranteed to have access to on a unix account. Mail uses a command line interface, so it is very clumsy for beginners. Still, it has a lot of power if you are willing to learn how to use it.
Eudora is a program designed for use on PCs with direct or PPP connections. It takes advantages of the Windows GUI, so it has the buttons, drop down menus, and other features you would expect in a Windows program. It also has features built in to grab messages from a mainframe account if you are attached via a PPP account.
Netscape is a web browser, but the later versions also have a built in mail program. The mail program is similar to Eudora. It takes advantage of the GUI environment. It is available for a number of platforms.
There are of course many others.