I'm having trouble finding the official home of the MercurialEclipse plugin (is there one?), and I have a weird feeling there are actually multiple projects by this name.
Here are some of the projects I've found:
See also this ticket I filed: http://www.javaforge.com/issue/23290
Since Intland Software (owners of JavaForge) are developing and maintaining the plugin, the official web site is http://www.intland.com/products/mercurialeclipse/overview/, and their official project page http://www.javaforge.com/project/HGE.
The BitBucket page stems from the time when the plugin was developed by Vectrace, but they transferred the project maintenance to Intland, so it’s no longer relevant.
Regarding the Google Code page, see http://www.jroller.com/andyl/entry/how_open_is_mercurialeclipse — At some point in time Intland started requiring a login to JavaForge to use the plugin, which provoked a let’s say... vivid response. However they reverted this behaviour quickly, and as Intland has contributed so much to the project, I forgive them. Before they jumped in, the project had stagnated. In the meanwhile one of the original Vectrace developers of the project created this Google Code project though. If you are sympathetic to that, I guess you could consider the Google Code project the official repository.