<p>My quick search reveals the reference implementation (<a href="http://stax.codehaus.org" rel="noreferrer">http://stax.codehaus.org</a>), the Woodstox implementation (<a href="http://woodstox.codehaus.org" rel="noreferrer">http://woodstox.codehaus.org</a>), and Sun's SJSXP implementation (<a href="https://sjsxp.dev.java.net/" rel="noreferrer">https://sjsxp.dev.java.net/</a>).</p>

<p>Please comment on the relative merits of these, and fill me in on any other implementations I should consider.</p>

Interesting to note that:
SJSXP performance is consistently faster than BEA, Oracle and RI for all
      of the documents described here in this study. However, it lags behind
      Woodstox and XPP3 in some document sizes and in the best cases, exhibits
      similar performance compared to these two parsers.
Article from Sun:
Streaming APIs for XML parsers