A SPARQL Endpoint is a SPARQL protocol service which enables users to Query a Knowledge Base (like DBpedia) with the help of the SPARQL Language. Simply, a SPARQL Endpoint is an Interface towards a Knowledge Base.
Some popular SPARQL (1.0) Endpoints are
General Purpose [http://sparqlorg/sparql.html]
Dbpedia-Live [http://live.dbpeda.org/sparql]
DBPLP (Computer Science Bibliographies) [http://www4.wiwiss.fu-berlin.de/dblp/sparql]
Linked Movie Database[http://linkedmdb.org/sparql; http://linkedmdb.org/snorql/]
CIA World Factbook [http://www4.wiwiss.fu-berlin.de/factbook/sparql]
OpenLink Virtuoso [http://lod.openlinksw.com/sparql; http://dbpedia.org/snorql/] This instance has the most computing resources behid it and the least restrictive "fair use" rules.
Europeana [http://europeana.ontotext.com/]
A SPARQL 1.1 Endpoints can be found at Ontotext [http://europeana.ontotext.com/sparql].
There exists an overview of all active SPARQL Endpoints [http://www.w3.org/wiki/SparqlEndpoints] as well as uptime service that monitors the availability off all SPARQL Endpoints that are cataloged in CKAN [http://labs.mondeca.com/sparqlEndpointsStatus/].
