I guess it depends what you need.

For personal finance account software, I can certainly recommend iBank from iggsoft. For writing and research, I couldn't do without Scrivener. I use Pages from iWork (Apple) for writing and Keynote for presentations, along with TextWrangler for HTML work and Yummy for FTP.

There's also ClamXav - Macs are very unlikely to get viruses and I believe that there are none in the wild, but one can never be too careful.

The estates use some heavy duty database and finance software, but that's no use to you - or me, in day to day life.

As for widgets, a must have is iStat but there's so many, it will entirely depend what you like.

If you click on the apple icon in your menu bar (at the top left) you will see an option that takes you to Mac OSX Software, which has lots of recommended apps and widgets.