I'm a big fan of software that is actually useful. The rest is just clutter, and most of it falls into the clutter category. Marc Andreessen covers his OSX killer apps of 2007, and has some pretty solid suggestions.
I agree with:
Netnewsire (though its slowly getting replaced by google reader) VLC (couldnt live without it) Ecto (though eventually I would just use Google Docs to publish) Adium
I Disagree with:
Twitter (needs larger user base, integration with facebook is a step) Flickr client (see my post about uploading pics) Cocoalicious (i like the concept, but it never worked well)