I've had some others in the past, but once I got Gmail, it's all I use now for free email (personally).
Yahoo can support POP3 mail access via Outlook, but it requires you buy their paid version - still not terribly expensive, but you have to pay for it. MSN used to be that way as well (hotmail), not sure if they still are. Also, if you ever intend to forward you email, Gmail allows you to do whatever you want with it - forward to/from any gmail account, I've not noticed any limits at all so far. Ease of use is wonderful, and the ads are very minimal and can be disabled altogether. I've gotten people to switch to Gmail who used Yahoo, AOL, etc, for their email client, and I've not had one yet who didn't like Gmail better, or at least as well.
I'm not saying the others are bad, but if it were me, I'd just stick to Gmail.... unless I wanted to set up my own personal email server, which I don't... for now. I have heard from some people that they prefer the Yahoo! interface. Also, don't forget Microsoft's Live email accounts... if you create a Microsoft Live account, you also get a free email address with that. I don't know if it works with hotmail servers or not.
There are limits to any/all of them, but Gmail pretty much feels limitless on capabilities (via plugins/addons) and freedom to choose. You can use POP3 or IMAP, you can forward to other accounts... you can use your Gmail account to receive other accounts, and even send from other accounts... although it will still show the "on behalf of" to do so.
Anyway, I'll hush. I just had lunch, and my typing at the moment is distracting me from my office hunch back-pain.
I was just telling a coworker this morning that I think I'm already gettting "old man itis" (I'm in my early 30s).