Safari 4 beta - very good browser, like the new tabs concept
iTunes - phenomenal music program
Last.fm - really useful music scrobbler/player. Put it on recommended and it's like a psychic, hostless radio station. Also has an iPhone version.
Airfoil - A cross-platform software implementation of AirTunes, for OS X, Windows, Linux and the iPhone.
Ableton Live - Really powerful music composition/performance tool.
Logic Pro 8 - One of the best DAWs I have ever used; far less complicated than Pro Tools or Cubase, whilst more intuitive and creative.
VDMX - A mind-blowingly cool, totally customisable and configurable visual performance tool. Accepts OSC commands, which can be useful
Quartz Composer - A free very powerful tool used to create a multitude of things (I use it to create visuals, but it has many uses!)
Adium - Because who would use Messenger for Mac? Love it's customisability.
Skype - Don't really need to explain this one

iPhone version too!
Mail.app - Never really got the hang of this until Hotmail enabled it to play nice in a pop3/smtp format. Very intuitive.
OpenOffice - Free alternative to Office or iWork (Though I do really like iWork.. might buy it!)
Address Book/iCal - love how this can sync with my iPhone, keeping all of my contacts and dates visible at home or out and about on my phone.
EventSync - ties into Facebook and copies your events you're attending into iCal, very useful.
VirtualBox - Free alternative to Parallels, works just as well.
iMovie - pretty useful free movie editing app, can't save at 1920x1080 which was a little disappointing though
VLC - the OS X version is pretty cool
Time Machine - brilliant idea, though arguably Linux has had this for ages (and Windows 7 seems to have a basic implementation as well) luckily I've not seriously needed this yet, though it can be pretty cool as you can format and reinstall from a Time Machine image.
iStat - most of you have it, so it doesn't need explaining
(For the iPhone)
TouchOSC - OSC host controller for the iPhone which lets you set and receive OSC controller data wirelessly. Very cool when coupled with VDMX for example (for example - controlling your entire visual performance from your iPhone - wirelessly!)
Remote - controls iTunes wirelessly
