Colours and other shiny things:
Color Scheme Designer - Play around to find a scheme you like or enter an html# and it'll give you complementary matches. This is one of my most used online apps!
COLOURlovers - Very useful for determining colour schemes and for seamless backgrounds.
Flickr - If there is anything visual about what you do, you should be using Flickr, another social media tool and a whole other community.
Wordle - A fun little online 'toy' for generating 'word clouds' from text that you provide.
Prezi - A very different kind of presentation software. It's not likely I'll ever use Powerpoint again.
Research and productivity:
Google Insights for Search - Compare search volume patterns across specific regions, categories, time frames and properties.
Google Alerts - An easy way to monitor your online reputation and a whole bunch of other things. Another one I make a lot of use of.
Doodle - A cool little online app that you can use to coordinate decision-making among a group of people... and it's really simple!
Join Me - A no charge online meeting service that allows other people to see your computer screen as you demonstrate, teach or collaborate.
Quora - An interactive web community devoted to answering people’s questions on a variety of subjects.
Ideas Checklist - A handy checklist to make sure your ideas are 'sticky' from Chip and Dan Heath.
Blacklist Check - MX Toolbox is a handy tool to check in with periodically to make sure your domains haven't been blacklisted.
WHOIS Directory Search - If you lose track of your domain, find out where it is by using a database tool. This is the one I use.