I personally would say "to sit at the computer", but a friend of mine said he heard "to sit on the computer" from a native speaker. That does not sound right to me at all, and I trust my guts, but still, Google results for "sit on the computer" are quite numerous (even more numerous than "sit at the computer"), and I wonder if on sounds right to native speakers out there.
Thanks for your input!
Best Answer
One can be "on a computer", which means one is using it. In order to do that, one sits at it.
To sit on a computer does actually imply sitting on it.