Why does Samsung Kies tell me that my Galaxy S GT-i9000 has no OS or firmware updates even though there are downloads available on the internet?
Because the updates that Kies offers you depend from whom and where you bought your phone. If you bought the phone in a shop, you purchased normally a country or region specific version. These versions get the update from Samsung one by one. For the Galaxy S you can lookup the release status here.
If you purchased your phone together with an carrier contract, chances are that you have an branded firmware. Then it's the carrier decision when to release an update. You normally also get a reduced set of functionality on branded firmware, like disabled tethering.
The firmware images that you find on the net are mostly leaked pre-version of the upcoming update.
If by "downloads available" you mean a OS upgrade, that is because those are not official releases, and since Kies is an official Samsung application they do not provide unofficial releases.
Currently, the last official release for the Samsung Galaxy S is Android 2.2 Froyo.
If you are referring to "official releases" and your OS is < 2.2, then you can have some variant system, for which there is no firmware upgrade, or you can have some problems with a not-up-to-date Kies version.
Also, check this thread out!