Infinite loop spaces. Various things by May. See the file “Addenda” for a brief guide to these.
http://mathoverflow.net/questions/17569/a-model-structure-on-symmetric-monoidal-categories
A brief but nice introduction is in Weibel’s obituary on Thomason. He explains what is meant by infinite loop space, connective such things, and group completion.
http://mathoverflow.net/questions/83406/weak-operad-and-deloopings
Thomason papers:
nLab page on Infinite loop space