We have accumulated a few handy online tools and links specifically related to Travel-SE so it might be a good idea to have them saved and updated somewhere for quick reference. Feel free to edit the CW answer and add your own link, tool or thingamajig.
Air travel sites
- Matrix Airfare Search
Similar to SkyScanner but less well known. Finds more cheap flights than a typical aggregator but doesn't sell tickets.
Can take a list of places and generate a map with each place pinpointed. Could be very useful for answering questions about multiple destinations in a certain area.