There are numerous online sources of information for Scottish Country Dancing. A primary source is the Royal Scottish Country Dance Society web page. Included on their web site is a list of “core” dances defined by beginner and intermediate dancing skill.
There are very useful resources for lots of Scottish Country Dancing stuff, including formations, footwork, timing (phrasing), and a dictionary of terms. Try http://www.scottish-country-dancing-dictionary.com. This website also provides briefing notes for lots of dances.
Another resource, as yet somewhat incomplete, but with lots of interesting stuff is the Scottish Country Dance Database. Definitely worth a look if you want to learn a formation or find what dances include that formation. Some music and video links, but not for all dances. Also, some crib notes are available.
A source for briefings of more than 3500 dances is Minicrib compiled by Charles Upton in the UK. Version 207 (in Word) is cited here, although there are frequent updates. There is also a Mac version available.
If you are looking for a particular dance, whether by name, author, music, etc., a useful source is My Strathspey. You will need some key information about the dance to use this site effectively.
Also, if you know the name of a dance, http://www.cardcribs.org provides abbreviated “card-size” versions of many dances.
For a very lengthy list of dance books, including some Scottish Country dance books, the American Library of Congress has a web site that you might find interesting to browse.
There are lots of other web sources for interesting and useful information about Scottish Country Dancing. Many clubs around the world have some great stuff on their sites. Here is just a sampling of what is available.
The Edinburgh Scottish Dance Group.
The Strathspey Server.
Would you like to see some beautiful dancing by the London RSCDS demo team? Check out this link – go to the video at the bottom left corner of their set of videos. They dance an 8 minute medley of dances in this video (turn on your speakers). Try to figure out some of the dances in the montage that they are performing. (Hint: One is the Inimitable Derek.)