prefix.cc is a very handy little service that I use a lot.
If you work with RDF a bit, you get to know the commonly used prefixes for ontology terms, such as rdfs:label, foaf:Person, owl:sameAs etc.
While I can remember the most important predicates and classes from these ontologies, it’s rare for me to be able to remember accurately what the full namespace is.
prefix.cc, created by Richard Cyganiak from DERI, lets you type in the prefix, e.g. owl and returns the full namespace (e.g. http://www.w3.org/2002/07/owl#) that you can use in a PREFIX definition in your SPARQL query.
It also has a simple API.