I know that there are some sites like Sigma Aldrich and AIST for $\ce{^1H}$ NMR and $\ce{^13C}$ NMR, but was wondering if there were any free sites to compare $\ce{^31P}$ NMR spectra?
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There are quite a few it seems, a couple of examples are below:
University of Wisconsin Phosphorus NMR data, starts with a shifts overview.
Stanford's Magnetic Resonance Laboratory site P NMR at the SMRL, provides comparisons and a protocol for testing.
A general NMR site is NMR Wiki - has many links to many sources.