You may also want to collect information for your own purposes and for this you can write your own specialised Twitter bot from scratch or use a library provided by kind souls.
Google provide some help by providing an infrastructure for you. Using this you only need to code the logic rather than the whole thing yourself. It interacts with the Twitter API respecting the terms of service, and manages the enrollment of followers. The twitter account acts as the bot. It's built on the .Net 2.0 framework.
There are other bot code resources available in Google code, here are some nice ones:
Python-Twitter - python wrapper for the Twitter API: allows people to connect via the web, IM, and SMS.
TwitterBroadcastBot - in Ruby - broadcasts a message whenever a friend update contains a certain word. Good for info flagging.
ContactsNearby - ASP.Net & InSTEDD- allows you to find out the geographical location of your friends on Twitter. Plugs into Facebook too.
MadCow - python - has a whole host of features for you to use like tracking bookmarks, ip lookup, and silly things like ASCII art and loads more.
Nathan at Flowing data has a nice tutorial on how to build your own Twitter bot, take a look - he also provides some code to get you started.