How many earth-sized planets have been discovered outside our solar system? Is there a combined registry of them anywhere? Where might I look for more information?
There are a couple of resources for you to look over the list of CONFIRMED extra-solar planets.
Wikipedia is always a nice starting point. Here is a list of the currently 53 known planetary SYSTEMS. And from that page, you can check out numerous other exoplanet details.
The definitive site though is http://exoplanet.eu/, which is touted as The Extrasolar Planets Encyclopaedia (The Interactive Catalog probably has the data you are looking for.). I would say that there are other sites as well, but this one is public and interactive. Many other sites require membership in the IAU for instance, and others don't seem to have been updated in a while. EDIT: Just found a nice table by The Planetary Society.
Since our ability to detect earthlike planets hasn't been that great until Kepler, and the results from Kepler are preliminary, maybe checking out the Released Kepler Planetary Candidates would be of value.
In perusing both of those sets of planets (555 confirmed, and the thousands of Kepler planets), I ran into quite a few that are earth SIZED although many of the tables are still sorted using Jupiter Mass as 1.0 (so you need to find planets near 0.00315 Mj).
Notice that I really stressed the word "confirmed" at the start. Right now we are still sifting through the data. And since we have a new method of reliably finding them (Kepler using the transit method), we have a lot of verification to still go through. Some specific exoplanets of interest as an answer to your question are listed on this Planetary Society Page. (Although I feel that The Planetary Society may be overstating some of the information about each of the planets.)
Although, if you look at The Extrasolar Planets Encyclopaedia for COROT-Exo-7, you find points of data that don't fully support the statements by the Planetary Society. i.e. the mass of the planet is not known with certainty yet, ranging anywhere from 2.8 ± 1.4 MEarth to 6.9 ± 1.4 MEarth. So the following statements by the Planetary Society may be a bit more self serving, but still exciting.
Overall, I would suggest The Extrasolar Planets Encyclopaedia as the more impartial source.
EDIT TO ADD: On the 20th of December (2011), NASA announced that there are more earth sized planets out there with the discovery of Kepler-20e and Kepler-20f. Keep in mind, as we start figuring out more and more about the universe, we'll be getting these type of announcements more frequently. I suggest you keep an eye on the Kepler Mission page as opposed to this answer, since they are more likely to be up to date. Note that this discovery adds a criteria to the significance of the planets. That is, they are earth sized AND they orbit a sun-like star. Hence the excitement about them.