Hardly essentials but I use these all the time:
JKDefrag, best and fastest defragmenter available.
File Commander, orthodox file manager, blinding fast, GUI equivalents just suck, been using it since 1995.
CrystalCPUID, handles CnQ on my overclocked uP.
Speedfan, de facto standard for hardware monitoring (i.e. all the rest suck more than this one)
Foobar, music player, everything else feels bloated and usability-impaired.
Irfanview, great viewer with batch support but also does basic processing tasks.
Google Chrome, use it instead of Firefox for all the JS heavy sites (including Gmail), until 3.1 is out.
ClamWin, offline virus scanner, use it once weekly just in case.
PuTTY, great telnet/ssh client.
UltraVNC, fast, full-featured VNC client (remote desktop)
Extensions for Firefox would need a thread of their own.
LFS related, LFS Replay Analyzer by wsinda and VHPA by Bob Smith.