Leon asked for:
Some program to fetch the mail from the isp. (eg. fetchmail?)
retchmail, getmail, fetchmail, or quite a few mail clients actually can do that themselves.
Something to read it and write replies. (I use pine!)
PINE isn't free software, so it's a pain to get fixes for it.
For text-mode fans, there's elm, mutt, nail, mailutils and there's a MIMEy one. For GUI, I use GNUMail (GNUstep) at the moment, but I think kmail (KDE) and evolution (Gnome) are also popular. I'd avoid Thunderbird until they sort out whether they want to be free software or not, but that's me.
A way of sending the mail when the box is connected, and a way of storing mail until the box is connected.
Pretty much any SMTP server software will do, but nullmailer is designed for this. A little buggy last I tried, though.
Maybe something to kill spam.
popmail will do simple pattern-based deletions. dcc can detect bulk email pretty well, but remember to whitelist your mailing lists.
A web browser.
Gecko-based browsers are the popular ones at the minute. Firefox, Mozilla, galeon.
Another view is the kHTML one: konqueror for KDE.
There's dillo, but I think that's currently having a painful move from GTK to FLTK. There's links which can do graphical. Some sick freak added image support to w3m, even. I don't think any of those support CSS yet, though.
Hope that helps. More at http://www.alug.org.uk/contrib/?InternetSoftware maybe.