Bitbucket charges by "private users". You can host as many open source projects as you want--many forks of the Linux kernel, for example--for free. However, if you want a private repository, you can only give read/write access to 5 people with the free account. https://bitbucket.org/plans
That's why I use it for all my projects--I have a Plan 9 kernel fork, an Inferno OS fork, and a bunch of smaller projects for a total of 13 repos, on the free plan.
Even though I prefer mercurial to git, Bitbucket could get a lot of converts if they started supporting git. Bitbucket is not the serious competition that I'd like to see Github get. For developers that prefer Hg to Git, Bitbucket has to compete with Google Code, Codeplex, and others...
fwiw, github doesn't really care about disk space for free plans as long as it's not being abused.
i was hosting the source code for my android rom on bitbucket because i would have had to pay for a big github account (over 1gb of code in 166 repos). i emailed github's support team and they said it wouldn't be a problem, so i moved everything to github.
That's why I use it for all my projects--I have a Plan 9 kernel fork, an Inferno OS fork, and a bunch of smaller projects for a total of 13 repos, on the free plan.