Question: Where can I find a good Eclipse Mercurial Plugin?

Answer: 
**[MercurialEclipse](https://bitbucket.org/mercurialeclipse/main/)**

Update site: <https://bitbucket.org/mercurialeclipse/update-site/raw/default/>

[Installation manual](https://bitbucket.org/mercurialeclipse/main/wiki/Installation%20and%20Configuration)

As stated below (in comments) the name is MercurialEclipse. The name HgEclipse was a fork that Intland did and was later "merged" back again with MercurialEclipse, which moved around a bit between the current site, Google code and javaforge.com.
Here is the web-site: <http://www.vectrace.com/mercurialeclipse/>  

The update site is: <http://www.vectrace.com/eclipse-update/>