Check out this little review I made aages ago - Fring and Barablu are the best.
http://forums.overclockers.co.uk/showthread.php?t=17712228
/edit I would also add Coreplayer to the list of essential apps, however I hate Coreplayer, they are full of carp - they said wmv would be integrated..almost a year ago now and they still havent. Nevertheless there is no other Symbian app that allows you to play as many different types of video codecs (without convertion too) as Coreplayer.
I'd also say MGmaps as this allows you to use Microsoft live maps which are better than Google maps (the maps not the app)
Also MOSH is a great p2p Nokia app that allows to to download free apps on the fly - there is some good stuff on there.
Lastly have you checke out Symella - download it and see what it can do..