That would depend on the species and the microarray platform used. For instance, there are some specialized on Arabidopsis using Affymetrix: http://affymetrix.arabidopsis.info/narrays/experimentbrowse.pl
You could try GEO Datasets at NCBI, just introduce the search term and you'll get lots of related microarray data. Here for example, using "cancer" as the search term:
http://www.ncbi.nlm.nih.gov/gds/?term=cancer
Datasets can be downloaded and in some cases some basic analyses can be done at the site.