BioMart has already been mentioned. It can do much more than ID conversion but it is very useful for conversion purposes, it is regularly updated and you can select different genome builds and all kinds of genomic features. It seems to me that you wish to retrieve GeneIDs linked to Affymetrix IDs. To select these attributes in BioMart: go to the Martview page to start a new BioMart query.
Select attributes on the attribute page: The Ensembl GeneIDs and Transcript IDs are default. Ensembl GeneID and Affy IDs are under the "External" tab. Select your chip there.
To limit to those genes which are on the chip, use the Filters->Gene menue. You can limit the genes to those present on various platforms or your favourite set.
There is an URL button in biomart that allows to retrieve a URL for your query and to pass it on to others. Try this example:
BioMart URL URL, that should be a good starting point.
If you are interested in KEGG identifiers (Pathways, Genes), EC-numbers, etc. the
KEGG Identifier page could be handy, because the KEGG ids are not in BioMart as far as I know.