Since you didn't specify user oriented software... within each of these project's "markets", they are definitely "worth the fuss".

Apache
SpamAssassin
ClamAV
OpenSSH
OpenSSL
BIND
Samba
Kerberos
OpenLDAP
Jabber

Since you mention Linux, I would also argue that FreeBSD is worth the fuss, but less so than 10 years ago (during the late 90's, and even the start of this decade, FreeBSD+Apache was _THE_ web server platform -- MS sucked on all levels, and Linux didn't scale in memory performance well enough to be used by anyone but a wannabe; the BIG sites (porn sites, for example) were largely standardized in FreeBSD and Apache); back in those days, throwing a couple million hits per minute at a Linux+Apache site would take it down quickly and HARD ... throwing it at a FreeBSD+Apache site wasn't even noticeable).

I would also mostly argue that OpenOffice is worth the fuss, but it's a bad example. They're not a leader (like Apache is a leader in web server design), they're probably the most annoying and glaring example of the anti-FOSS "it's all just imitating proprietary software" criticism/stereotype.

McCafee started out as 100% open source (for the app; the signatures were "for pay"), but I don't know if that's still true. And I have no idea if they're still "worth the fuss" or not.

Other important 100% FOSS projects, though with a lower profile than the list above:
nagios
cfengine
tripwire
tcpwrappers
Sendmail
Exim
Cyrus
UW-IMAP
OpenAFS


That's just off the top of my head. Given more time, I could probably come up with more.