How best to use Twitter is a devloping debate. I use it for following tweets that provide practical, applicable advice that helps me to design and market websites for my clients.
Should my clients be using Twitter as a Marketing strategy? What should the strategy look like?
This guy has advice about how to use Twitter as a marketing tool (and he has a LOT of followers):
http://twitter.com/guykawasaki
I use these guys for helpful and periodic Web Design tips:
http://twitter.com/MintTwist
If you fancy it, follow me on twitter here:
http://twitter.com/alexis_mt