For meeting locals/travelers:
http://www.couchsurfing.org – the hospitality website (you find people to stay with or meet) with most users, although I prefer:
http://www.bewelcome.org – because it is open source
http://www.warmshowers.org – very good hospitality website for travelers
Other online community websites according to your interests.
Searching for flights/travel deals/transportation:
http://matrix.itasoftware.com/ – my favorite website for searching for flights. Google bought it and this is what google flight is based on but the original has more functionality. However it is not enough as it doesn’t show all options. Other flight search websites I like: http://www.dohop.com, http://www.momondo.com, http://www.skyscanner.com and the rest of the bunch.
Go to an airport’s website, see what airlines (including charter airlines) fly there and check their websites for promotions
http://www.hitchiwiki.org – hitchhiking info
https://www.lonelyplanet.com/thorntree – a good forum, sometimes the group discussions on couchsurfing are better though
https://www.openstreetmap.org/ – open source maps, OSMAnd is the best Android app for maps
http://teryrobin.blogspot.com/p/about-me.html – Bulgarians don’t travel enough but this girl is definitely an exception
http://www.yr.no/ – my favorite weather website
http://www.easypersian.com/ – Persian classes