Is it a common practice for reviewers to recommend their own papers in the review?
This is a delicate issue.  


On the one hand, a reviewer is most familiar with their own work, and will certainly know if their work is relevant to the paper that they are reviewing.
On the other hand, it can feel like a crass "citation for publication" trade.


The way that I typically approach it is to only recommend citation of my own work if I am also finding it appropriate to recommend citation of other related work at the same time.  

Put another way: if the hole in their literature review is "me," then I find it likely that I'm wishing they cited me because of personal bias and my own ego.  If the hole in their literature review is "me and a whole bunch of other things like me," then I find it likely that it's a legitimate gap and it's OK to recommend my work as part of the set of things to fill said gap.