I think Dropbox is a very useful tool especially when you want to work on stuff at home or work and access it from both locations without having to transport your files on an external hard drive or other removable media.
I found Dot Voting to be a very intriguing tool especially when doing professional development.
I have used jog the web in the past and have found it to be very useful. Jog the web is great when you want to demonstrate web 2.0 tools and need all your webpages in one space.
