General themes:

P. W. Anderson (1972).
More is different
Science, Vol. 177, pp. 393-396.

M. A. Bedau (1997).
Weak Emergence
In James Tomberlin, ed., Philosophical Perspectives: Mind, Causation, and World, vol. 11, pp. 375-399. Oxford: Blackwell Publishers.

Y. Bar-Yam (2004).
A Mathematical Theory of Strong Emergence using Multiscale Variety
Complexity, Vol. 9, pp. 15-24.

I. Couzin (2007).
Collective Minds
Nature, Vol 445, p. 715.


M. Mitzenmacher (2003).
A brief history of generative models for power law and lognormal distributions
Internet Mathematics, Vol 1, pp. 226-251.

M. E. J. Newman (2006).
Power laws, Pareto distributions and Zipf's law
arXiv preprint.

A. Clauset, C. R. Shalizi, and M. E. J. Newman (2007).
Power-law distributions in empirical data
arXiv preprint

Online writings and gatherings of writings:

The Complexity Digest rounds up articles from across the spectrum of complex systems.

Peruse Cosma Shalizi's notebooksfor a sober view of complex systems and all things related. See in particular:
Complexity Measures