Brussels South Charleroi Airport

The city's second airport
Word to the wise: Don't expect to find a particularly large selection of shops or restaurants at Charleroi, as it's only a small, low-cost hub.

If you're coming to Brussels on a flight with Ryanair, there's a strong chance you'll be landing at this airport due south of the city. Less convenient for the centre than the main airport, it's still a manageable hour away from downtown Brussels. With just a few airlines using it, it's also very small, meaning the business of getting off your plane can be mercifully short.
The easiest way to get to Brussels from the airport is to take the shuttle bus to Brussels Midi (South) Station. The buses run every 30 minutes (and every 20 minutes at peak hours) and costs EUR 12 one-way, EUR 22 return.