Rationalism should not be confused with rationality, nor with rationalization.
-Wikipedia article on rationalism
I frequently see people using rationalism in place of rationality. Usually other commenters understand them, however I believe that using the word rationality is superior. The Less Wrong tag line is "A community blog devoted to refining the art of human rationality". On the other hand, rationalism is the philosophical term for a very different epistemological position. Furthermore, -the -ism suffix has some undesirable connotations.
It could just be that its one of my pet peeves, and thus I notice it more, but I see it very frequently. In fact I wrote this post because I wanted an easy reference instead of having to repeatedly retype my explanation every time I correct people. Sadly, Google is refusing only return results that include the exact string "rationalism", so I can't give exact numbers.
For exact match searches with Google, put the string in quotes.