IS-LM model

I don't particularly like the politics of this one, by Nouriel Robini, but it is an excellent description of the IS-LM model by Hicks which even Krugman admires.  (Hicks himself somewhat moved away from it in his later years, and said it didn't really capture the essence of Keynesian economics, which is really about instability rather than equilibrium.)  The Wikipedia dscriptions of LM is terrible, this one gets very detailed.

http://pages.stern.nyu.edu/~nroubini/NOTES/CHAP9.HTM