GoodReader is a little on the pricey side, but it's arguably worth it. As well as including a really quite handy reflow function for PDFs, it can also read Office files (DOC, PPT, XLS), iWork, TXT, HTML and Safari, images, audio and video.
Business cards are so easy to lose. This Card Reader uses your iPhone's camera to scan the business card, read it and extract the information into a handy database so that all of your contacts are there and easy to find.
It isn't a substitute for an attentive secretary with a skill for shorthand, but Dragon Dictation does allow you to take voice notes, which it will then convert to text to be sent as SMS or email, or pasted into another application using the clipboard. You can even dictate Facebook and Twitter posts.
Whatever your reason is for wanting to check currency rates, XE Currency is fast and reliable, offering exchange rates on over 180 currencies. It stores the most recent rates on your phone, too, so you can even check them offline, and to update, all you have to do is give your phone a shake.
If you need to make credit card payments on the go, Mobile Payments is one of several apps that turns your phone into a credit card terminal. It's extraordinarily handy, but do your research; look up a few different apps and phone around providers before you decide on one.
LinkedIn is a good way to access your LinkedIn network on the go, but be aware that its functions are limited. There also appears to be a few bugs that need fixing, but no doubt the devs will get around to it.
Evernote exists for all of your note-taking needs: saving files, web clips, jotted-down notes, voice memos, photos and so forth. You can also download an Evernote app for your Mac or PC and sync it to your phone for access and editing. Be warned, though: go over the 60MB monthly transfer limit and you will need to pay for a subscription.
Dropbox is a simple and effective way to have access to your files anywhere, anytime. It syncs your file storage across all of your devices, and even does so securely on the web, so you can keep your important stuff handy wherever you are.
Bento is basically the grandmother of PDAs. You can store and organise contacts, make and track project and task lists, plan events, keep an expenses log and keep time sheets and staff lists, and has a whole bunch of other useful applications.
The Australia Post app won't let you send parcels, but you can calculate postage, find postcodes, track packages, locate postboxes and Australia Post stores and pay bills that you would usually have to visit the post office for.
Keep track of your billable hours with Daily TimeLog, which allows you to track how long you've spent on a given project or task, with notes and automatic calculation. You can then export the data by email with an attached timesheet.