I have changed my default browser from Safari to Firefox 1.5. Although I have had both installed on my desktop for a very long time I have always used Safari over Firefox 1.0 as it was slightly faster on my PowerBook. With the release of Firefox 1.5 these speed issues have been solved and with the added functionality of the del.icio.us extension (and the obligatory weather plugin) Firefox 1.5 is a nice piece of software.
My Firefox setup with extensions (click to enlarge)
A great webpage for OSX software is the Essential OSX Software listing. It is pretty comprehensive and covers most of the really great (and generally free) software utilities available for OSX.