Here are some apps I use or think might be useful as a Mac user (long list!). I use most of them.

Useful Applications for Mac OS X

Mail
iCal
Address Book
Stickies
iPhoto
iMovie
iMovie HD (old)
Pages (sorta)
Numbers (sorta)
Keynote
Microsoft Office 2008 (Word & Excel, mostly)

Third-party applications

Adium
Adobe Acrobat 8 Pro
Adobe Dreamweaver CS3
Adobe Illustrator CS3
Adobe InDesign CS3
Adobe Lightroom
Adobe Photoshop CS3
Adobe Reader 8
Aperture
Applejack (may not work with Leopard)
AutopanoPro
Azureus
Canon Utils
Capture One
Carbon Copy Cloner
Chicken of the VNC
ChronoSync
Coda
Comic Life
CrashPlan
Crontooie
Disk Warrior
DropCopy
eFax Messenger
FDRTools HDR
ffmpegX
File Juicer
FileMaker Pro 9 Advanced
Final Cut Studio
FireFox
Flexify
Flickr Uploader
Flip4Mac
flvThing
Font Finagler
FotoQuote Pro
Google Earth
GPG for Mac OS X
Growl
HandBrake
HBTimer
iMediaBrowser
iPodDisk
iTerm
Journler
LoadMyTracks
MacGPS Pro
MacTheRipper
Mail Act-On
MailTags
Missing Sync for Blackberry
Missing Sync for iPhone
MoviePod
MPEG Streamclip
NetNewsWire
Nocturne
Noise Ninja
NTFS-3G
OmniOutliner
OnyX
Pathfinder
PCalc
PDFpen
Photo Mechanic
Photokit Sharpener
Photomatic Pro
Pukka
Quicksilver
Region X
Sensus Manager
Senuti
Skype
Smugmug uploader
Snapz Pro X
SuperDuper!
Switch
TextMate
Throttled Pro (does this work in Leopard?)
Toast
Transmit
Twitterrific
UnRarX
VersionTracker Pro
VisualHub
VLC
VMware Fusion
WhatSize
WriteRoom
Yojimbo

Preference panes

Afloat
GnuPG
Growl
iStat menus
teleport
TextExpander