As some companies have figured out, there is a lot of information to be gleaned from Twitter that can benefit their business. They can manage reputation and customer service for example. There are however other reasons to collect information which do not involve marketing motivations but for example as I do, for research reasons. I study the language used, behavioural analysis and that sort of thing.
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.
– python wrapper for the Twitter API: allows people to connect via the web, IM, and SMS.
– in Ruby – broadcasts a message whenever a friend update contains a certain word. Good for info flagging.
– ASP.Net & InSTEDD- allows you to find out the geographical location of your friends on Twitter. Plugs into Facebook too.
has a nice tutorial on how to build your own Twitter bot, take a look – he also provides some code to get you started. 

Perl hackers can use Net::Twitter to access the API. I’ve used it in a couple of projects and I’ve found it’s great.
Excellent piece of info Andy – Thanks!
check out this service on the top of Twitter: http://t411.linkstore.ru
It is 411 services for Twitter