Tools I can't live without...

Audio

EAC - Exact Audio Copy (Copy your CDs to HDD)
FLAC (incl. in EAC)
foobar2000 (best audio player ever)
IsoBuster (in case EAC doesn't do it)
The GodFather (tagging/managing tool)

Development

JetBrains (Dev Suite)
Process Monitor
PSPad (for HTML/CSS/PHP/JS/etc..)

HDD

Bulk Rename Utility
explore2fs (explore EXT2/3 partitions with Windows)
HD Tune (HDD benchmark)
hkSFV (hash + MD5, oldie but goodie)

Network

FileZilla (FTP Client/Server)
inSSIDer (WiFi-Scanner)
PuTTY (SSH, Telnet, ..)
mRemoteNG (Multi Remote Connection Manager
WireShark

Office

Adolix Split & Merge PDF
Bullzip PDF Printer
Foxit Reader
OpenOffice

Video

AVIMux GUI
Handbreak
MakeMKV
VLC media player

Visual

IrfanView
yEd Graph Editor
ZoomIt (pause-timer for presentations, draw on your desktop)

MISC

7-zip
DEAMON Tools Lite
ImgBurn
KeePass (Password manager)
Pidgin (Multi-IM)