Tedium is a Twitter Digest Manager: it will watch your friends' tweets, and produce an emailed digest every so often. There's also a slightly nifty web interface, which gives control over things like which authors to pay attention to their replies (either to or from them) even in digest mode; it also gives training control over a spam filter.
Grab the source via subversion (svn co svn://svn.tartarus.org/sja/tedium), and run once on the command line to configure. Run with an email address to get a digest; there are some other options available in the usual fashion. It expects to be run as a CGI script behind HTTP authentication (in the way Apache does it), which gives some measure of protection against accidentally letting everyone see protected tweets and any security holes.
Currently the prereqs are a little weird, including jinja 0.9, which is what Debian stable ships with at the moment (and, incidentally and no matter what Jinja/Pocoo claim, has a much more sane API than the post-v1 rewrite). You also need spambayes, and sqlite with pysqlite bindings.
Some bits may not work properly on some browsers (layout in IE, probably the reply function in some as well); patches welcome. There's no support for DMs, and right now I'm not planning any as I'm not entirely certain how I want to use them in my life anyway.