Couchsurfing.org
While you can always get to know people by just living an active and open life, couchsurfing is a community where you can just go and say: "Hey, I'm new to this place and don't want to be lonely - who wants to hang out?". And everyone in the community is so damn open - I have without exception found that couchsurfers are always great people (maybe it has something to do with the readiness to let strangers into your house).
This has already been mentioned by WGroleau - but this site in particular I know works very well. The whole site of Couchsurfing in its core actually revolves around community and getting to know people rather than just finding a place to stay.
I am part of the couchsurfing community myself for the past 2 years, and I have spent more time just meeting other couchsurfers than hosting/surfing. Some examples of what I did with people from couchsurfing:
- In my previous town in Germany I met regularly with a group of others to practice french. We'd go out for a drink, go for a trip, see a concert, cook together... and attempt to spend the entire time in french : P We had a few French natives, and some people from other countries, a few germans - we'd meet and just have a good time together in a rotating group.
- I found my previous apartment via couchsurfing, and some couchsurfers helped with moving.
- I have a painter friend, who organized a private painting workshop - half the people who came were couchsurfing members from the city.
- I've made friends with people from couchsurfing in Belgium, France, Germany and Poland, in huge cities with millions of people, and in small cities with only a few thousand.
When I go to a new place, I always check out couchsurfing as a first place to find some people. My upcoming vacation in Vienna? "Hey couchsurfers in Vienna, who wants to spend a great time together?
