Firstly I wanted to mention a Mozilla Firefox extension called Zotero. Zotero is a web based research tool which allows you to manage, collect and cite research sources. Although this is not necessary for the average sound professional, what makes this fantastic is the ability to sync web sources. This is is extremely useful for users with multiple computers, or for workplaces who want to share "common sites". I have used del.icio.us for this same purpose before, however Zotero is far quicker, due to links living locally after sync.
DropBox is another app which I am tending to use more and more. DropBox is a cross platform + web based file sharing system. For free you are granted 2Gb of fully sync-able data! I have a folder set up on all of my computers and any documents or files which I always need access to are only a click away. DropBox also keeps a backup of your data on the web incase of any accidental deleting. This works just like any other folder in your finder.... except it can be shared and synced to anyone you wish - or kept private. Some uses I have had are: contact lists, app udates, picture updates, or just sending audio! This is truly one cool app!
Remote Pad an app for the iPhone and a computer which allows you to use your iPhone/ iPod touch as a remote touch mouse over a wifi network. Great for presentations!
What is great about the mentioned applications is the fact that they are all free!