As a 2nd grade teacher I say Pattern Blocks are my best manipulative. I find that my students begin to build relationships about parts and wholes just by messing around with them. The Marilyn Burns' activity of "Building the Hexagon" is one of the best activities I've ever done with the concept of fractions. 2nd graders catch on REALLY well to equivalent fractions-- especially with the Pattern Blocks. HOWEVER!! I do use a LOT of other stuff and I think it's really important to work with the set model of fractions as well as the area model. We did some work with recording probabilities with fractional notation this year, too.
AND by the way, thanks for your VERY helpful comments and question, Karen, I, too, have had enough of teaching styles (yes, I wrote about how I do it, too!) AND I do NOT want to hear that teachers stink ANYMORE ever again as long as I live. So there! On to a discussion of fraction manipulatives. :-) Cindy Chapman@apsicc.aps.edu