Here are some Macintosh-only apps I use a lot:

Scrivener - Writing

Mellel - Word processing

Adium - It combines all my IM identities into one application: Yahoo IM (multiple Yahoo IDs, actually), AIM, and GoogleTalk. So I just log in and monitor them all in one app.

Pukka - sends stuff to del.icio.us

Quicksilver - A task switcher and more

Cyberduck - FTP client

Omnigraffle - Creates diagrams

NovaMind - Mind mapping software

Paparazzi! - For grabbing a full copy of web pages to pdf or jpg

Twitterrific - Twitter client

Combine PDFs - self explanatory