How do I find the annotations (mappings) for Entrez, NCBI or other IDs?

FAQ tags: 

The list of e.g. Entrez IDs, NCBI IDs, should be converted to UniProtKB or model organism database IDs, and those IDs used to search the GO database.

UniProt and PIR have similar ID mapping tools to help with the conversion:

GO annotations from QuickGO can be filtered for many parameters and provide mappings to several ID, e.g NCBI or Ensembl gene IDs: http://www.ebi.ac.uk/QuickGO/GAnnotation

The GO website has a list of taxa and authoritative database groups: http://www.geneontology.org/GO.annotation.species_db.shtml