Windows Live Mail 2011 is actually a pretty good e-mail client.
It's part of the Windows Live Essentials that also has a Photo Gallery, Live Messenger, Movie Maker and other items, all free for the download. You can choose only the programs you wish to use.
You've noticed that Windows 7 doesn't come with a built-in mail client as past versions of Windows has. There are a few out there that you can download for free, including Windows Live Mail 2011 that WR2 has pointed out. I'm giving a +1 for his answer. You can download it at the link he provided if you want all the Windows Live programs. If you are only looking to download Windows Live Mail, you can get it here - http://explore.live.com/windows-live-movie-maker?os=other