The following are some websites I use when planning a tour. Do you have some you use, which you could add to this list?
Time and Date
I use the Time and Date site (http://www.timeanddate.com/) for several things, but one is to look at what holidays and observances are going on around the world. This infomation could be useful for cycletourists as well, so we don't get caught out if we're expecting everything to be open and supplies to be readily available on a certain day.
Holidays around the World
That site also provides some weather information around the world.
Airline Baggage Regulations for Bikes
Another one I post frequently here is the Airline Baggage Regulations for Bikes webpage (http://www.ibike.org/encouragement/travel/bagregs.htm). Although it may not always be right up to date (airlines change their rules frequently) it can be a good place to start when making decisions about which airline to choose.
When you've had a look over the chart near the bottom of the page, and have selected an airline, always look up the bicycle baggage information on the airline's website, and if you've got any questions about it, contact the airline.
And then check again within days before you go ... things can change.
Mile by Mile
Mile by Mile (http://www.milebymile.com/) is probably not an obvious choice for cycletourists because it is an RV site, but the Itineraries describe routes in great detail and post photos of the roads.
Sometimes they also list campgrounds, tourist attractions, and other things which cycletourists might find useful.
These can be a good resource when thinking about where to go for your next tour ... to give you some idea of what you might find when you get there.
Lonely Planet: http://www.lonelyplanet.com/
National Geographic: http://www.nationalgeographic.com/
Government Travel Advice/Information Sites
These sites have a lot of useful information for travellers. I would definitely recommend checking these before going anywhere. Your government might have a similar site as well.
Canada's Government Travel Information Site: http://www.voyage.gc.ca/index-eng.asp
Australia's Government Travel Information Site (Smarttraveller): http://www.smartraveller.gov.au/