This is a twitter client that uses the web api to pull updates and forward them to a specified email address, including the email address that sends messages to your phone as SMS. The primary purpose of this is to replace the direct device messaging that twitter has built into it, because twitter doesn't support message filters. Filtwit automatically adjusts poll delay based on twitter's rate limiting. Filtwit may also allow message mutation in the future.
The primary reason for creating this was to remove messages sent from twitterfeed and rss2twitter, which I consider spam, while allowing messages from other clients to continue.
The available constraints are all the fields that twitter passes in its XML structure. The source and a sample configuration file are available on my SVN server. One particular quirk of this program is that it will only forward messages that arrive after it has started running. This does NOT currently support TLS or SSL, and hence is entirely INSECURE even though it may be handy as hell.