One of the features that I wanted to implement in TwitteyBot was the ability to automatically reply to people who have certain search term. Unlike the normal updates that only reach the people who follow you, this could be noticed by people whom you are replying to, thereby having a wider, newer audience. Here is the scenerio.
Searching for a term like "#chromeextensions" would show all the people who are talking about Google Chrome extensions. It would make sense to reply back to them and maybe tell them about something like FlashPlus.
You can have your own bot by changing the parameters of this pipe, copying the "Get RSS" link and configuring it in Twitterfeed.
I created such a bot using YQL and Yahoo Pipes. Here is the pipe that spits out a feed that should be tweeted. Twitterfeed can then be used to convert this RSS feed to tweets for a specific twitter account.
The pipe uses this YQL to find all the tweets that have a certain twitter term. It then follows through a loop where the user is extracted and the advertisement line is added. This is spit out as a feed that is configured into Twitterfeed.
The only limitation is that this method has a limit of 5 tweets every 1/2 hour. I tried using Friendfeed to overcome this limit but friend does not seem to pick up the changes in the RSS feed.
If we make the advertisement text dynamic, we could have a bot that sends out answers from a we service. I created a @questionbing bot that tries to fetch the instant answers results from bing for questions sent to it. Here is the pipe that does this. Try asking it questions like [http://twitter.com/home?status=@questionbing how many days in a year] and you should get an answer.