Fax - where can I send one in Brussels?

Try any small newsagents. Many of them still place their daily orders via fax. I know that the three around where I live south of Waterloo all have fax machines specifically for that reason.
Also - any place that has an old photocopier. Most copiers installed in the 1990's early 2000's were also hooked up as fax machines at the same time. It was "the technology" of the time.
Amazing how things have changed!

Also any little shops that offer internet services, scanning, that kind of thing. There is one on Chaussee d'Ixelles (near the Flagey end - a shop that also had mobile phones in the window), but many others around the city if you look in the little streets.

Paper shops (papeterie) and copy shops often have fax machines as well.

If you're downtown Belgium Copy on Boulevard Anspach can help you.

Why not use an online Fax solution such as http://www.myfax.com

You can scan in whatever needs to be faxed and send from this site

For a small fee or if you buy a drink at your nearest hotel, you can always ask the receptionist to send your fax for you.

There is a copy shop in Stockel shopping centre where the metro is. I sent one recently from there.