[Zev]

Not really a company, but an open source project I contribute to (there's also an App Store app, so it might qualify as a company, I suppose) has a twitter account that we use. We mostly have a few twitter searches for anyone having problems with the app (Colloquy) or looking for an IRC client on a platform we support to recommend to them. Occasionally we'll give out promo codes, either to bloggers or to anyone following us/twitter's public stream to use.

Note, for this, we don't use a bot — A. because traffic is low volume, only a few tweets every few days and B. (imo), bots that send send the same message to everyone regardless of what the query is, is obnoxious.