The aircraft in DCS world can be expensive, so it would be nice if we could get them and play them, online and without any restriction for free...

This video will show you, and Eagle Dynamics (makers of DCS) appears to think so too, and for some time have offered every module for free for a couple of weeks, and are so confident we will like them, it's open to every module and map available in the game. What's more, after 6 months, we can get them all for free all over again. We can do them all at once, or keep a never ending chain of free trials on the go which is what I recommend until you've found all the ones you like best.