I currently use a Mac Mini (1.5ghz G4/1GB RAM/80GB HDD/Combo/AP/BT) running Mac OS X 10.4.3. I like it because it is cheap, small, and almost totally silent. It has been running non-stop since 10.4.3 was released.

My favorite Mac application has got to be GraphicConverter. It is an incredibly well supported application that I use almost everyday.

A close second would be SubEthaEdit, which is a great text editor even outside of collaborative editing. I have come to prefer plain text for my documents, which works well with my preference for monospaced fonts.

I would also have to agree with Ihnatko's opinion of OmniOutliner. I bought a license and I love the program, which I am using to track my comic book collection.