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.
It probably doesn't work any more.
If you want to play, and are prepared to fix anything and everything that no longer works, the source is on github, 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.
You need pysqlite2, jinja2 and spambayes installed, although I have no idea which versions will actually work any more.