Any websites that list writing groups by geographic location, or genre?
Should I try libraries or something?
Fan forums of a favourite author?
Are there "virtual" writing groups (by email or chat or whatever, as opposed to in person)?
I've heard about "virtual" writing groups; I have no experience with them.
For meeting other writers in your area:
Just search for your location and your interest (in this case writing).
Libraries and other community spaces often offer writing workshops so it might be a good idea to look there too.
I second The Meetup.com recommendation, but would like to add that participating in NaNoWriMo has always seemed a great way to meet up with other writers in your area and online.
You can go to critiquecircle.com and find reading groups there. You can find people there who will read your manuscript, either complete or in parts, and offer feedback and suggestions to help improve your work. You can also go to Facebook and ask to join a group there called "Indie Writers Unite". I belong to that group and there are a lot of active writers who offer advice and volunteer to be sounding boards for other writers. I know several who have formed their own reading groups as well out of that group.
If you live in or close to a big city, especially one with a big writing scene (theatre or novel), there should be many writing groups. Besides Meetup as has already been mentioned, there may be online local forums (Chicago used to have ChicagoImprov.com which was a forum for improvisers and sketch writers/actors).
Taking a writing class would also help you meet other writers and join a group or create your own.
Checking with your local library is a good option. They may sponsor a writers' group; even if they don't, the reference librarians should be able to help you find a local group. They often work with local writers, and many library employees are writers themselves.