I miss the old discussion forums. Had some amazing times on them, and met great people through them that I'm still in touch with.
Some forums still exist:
wrongplanet.net
foreverdelayed.org.uk (or was it .co.uk?)
groundhopping.nl
namepros.com
tweakers.net
userbase.be
webhostingtalk.com
emaildiscussions.com
However, the majority have ceased to exist and turned into Facebook groups instead. I much more prefer a real forum. Like this one.
Facebook groups are harder to navigate through and follow up on posts you find interesting, and the layout is not as nice as a real forum.
PHPbb forums were really popular at the time. They're still around, but you have to search for them.