jailbreaking your iphone isnt to everyones liking. but if it is then here are my recomendations for the best jailbreak apps to get:

mywi
it turns your iphone into a mobile wifi router. excellent if you have a laptop or the like and need some access
when you are out and about. think tethering without a cable.

lockinfo
puts calendar, mail, weather, twitter, etc on your lock screen

nmap
yes, nmap on your iphone. as useful as nmap on anything
else :)

winterboard
gives you themes amongst other things

apt-strict
lets you do apt-get update && apt-get upgrade
from the commandline without loading the (very) bulky and slow
cydia

there are loads of
other excellent jailbreak only apps, but with these installed your
iphone will be much more usefull :)