As was pointed out awhile back, BoxOfficeMojo was purchased by IMDb and for some time the web address just redirected to IMDb. However, it now appears to be back up online.
No idea what was going on behind the scenes between IMDb and BoxOfficeMojo. Maybe IMDb's managers were in the middle of some kind of negotiation with the writers at BoxOfficeMojo and pulled their site as a kind of pressure. Maybe it was intended to be a permanent shift to IMDb but there was a backlash that persuaded them to reinstate the site as a separate location. Or maybe it was just some kind of technical issue they had to fix. Who knows? (Well, they probably do.)
In any case, I'm glad it's back. I don't always agree with BoxOfficeMojo's qualitative analysis, but there's no denying their quantitative collection of movie data is valuable. I've long lamented that there's nothing quite as reputable or authoritative for the same purpose in the games industry (as there is for television in TV by the Numbers).
EDIT: Oops, embarrassing titling error. Fixed now.