Public endpoints with query editors give beginners the chance to write SPARQL queries without installing software or loading triples.
A lot of people use the DBpedia SPARQL endpoint, but DBpedia is a large, complex and rather dirty data set... If somebody doesn't get the results they expect, they don't know if it's the data or the query?
Is there something out there which is easier to use, that's similar in nature to, say, Microsoft's Northwind database? What's the best URL I could point to beginners which would have them writing queries quickly?
The Linked Movie Database (http://www.linkedmdb.org/) is good for fun queries for beginners. It doesn't have the depth of data of DBpedia (or of IMDB), but the data is pretty consistent as far as I can tell.
I rather like FactForge. It contains a mix of data(sub)sets from different sources (including DBPedia, Geonames, CIA world factbook), which makes it a nice playground, and its online query editor offers quite a good list of example queries to start with. It also offers an explore interface which can be used to figure out the structure of the data.