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 what it's worth, I've also quite often seen (and been mildly annoyed by) the same thing, mostly in Eliezer's writing. E.g., in phrases like "your strength as a rationalist".
If it weren't for the other meanings (denoting philosophical positions) the word has had, it would be quite a useful word to have -- a rationalist being someone who practises rationality, as a physicist is someone who practises physics and an artist someone who practises art. But, given the existence of those other meanings, in Eliezer's writing it's never perfectly clear when he means something like "person who practises rationality" and when he means something more like "person who gives precedence to rationality in all things". (One could argue that the two are the same thing, but they can't be made the same thing by definition, so it's better to have two words.)
(I don't think Eliezer ever uses the word with the older philosophers' meaning found in the SEP. -- reason versus sense-data, rather than reason versus unreason. To a good first approximation, neither does anyone who isn't a professional philosopher.)