There are probably way to many to state here. I'd have to state as I have many times before. The most used and most useful app I rely on is,
Konqueror from the KDE team.
It's the best file manager / web browser I have used.
Even though I am a GNOME user, i still do all file management via Konqueror.
As for reliable. Lets just say I recently upgraded my system with Slackware 11, which came with KDE 3.5.4. No sooner had I got the system all updated when a new release came out. Lots of improvements and bug fixes.
I just started using this new music manager - AMAROK. Since I could not get the mp4 plugin to compile for XMMS. So I tried Amarok.
Even from within Konqueror, the filemanager / web browser. I already have a toolbar for Amarok.
There are far way too many goods for Konqueror.
Download manager included, Console, CVS frontend...