The Desastersoft campaigns are excellent, they make the most of what the game has to offer at the moment, worth buying in my opinion.
This is the best place to go for free campaigns:
http://airwarfare.com/sow/index.php/...ry/3-campaigns
Heinkill's free German invasion of the UK missions are very good:
http://airwarfare.com/sow/index.php/...n-packs-lw-raf
RedToo.