This sites got everything and then some. Basic shooting tips, more information than you can shake a stick at when it comes to creative masking, multi exposures, developing film and pushing and pulling your images. There is a whole section dedicated to trouble shooting, and while us Holga users acknowledge running into troubles is half the fun, its nice to have a little bit of help.
Square frog is a wealth of information for those just getting started however I'm certain even the most experienced Holgographer (try saying that three times fast) would find some useful info here, If not a wealth of inspiration... as their flickr pool squarfrography has over 11,000 individual shots - that's a whole lot of Holga love!
The shot above was taken by a flickr devote who goes by the name faster you can view more of his work here