Windows Live Writer
Part of the Live Suite available free of charge from Microsoft. This is a useful piece of software for writing blog posts offline (and publishing them to your website).
Google Calendar (I’ve tried lesson planning again this year using Google calendar. I kept it up for a over half a term but still ended up going back to a paper diary). The SMS reminders are useful for reminding me of appointments when I’m not near my email.
Twitter. I have a PLN (personal learning network) on Twitter but I’m still not convinced that this is a must-have tool. A significant majority of people I follow are broadcasters rather than being interested in a two-way conversation, and most don’t reply to tweets directed straight at them.
What software do you use every day? What software couldn’t you do without?