Let me highlight some interesting tools to do it.
The very first thing to use, if you didn't already, is to leverage Google Alerts. Set a few agent that will bring you back on the fly, daily or weekly, whatever web page, news item and now blog posts relative to the keywords you're looking into. Here is a result of blog search for Marketing 2.0. I use daily agents for the core topics I am covering for Sun and have an e-mail sent to my inbox and provide my team around the world with a weekly summary. Very powerful to always stay on top of things.
Now what about discovering your competitors marketing campaigns? Spy Fu can just help you do this for free. It is still in beta but effective. Enter your competitors name in there and find out how much they're paying for search advertising daily, the number of total clicks they're receiving, their keywords ranking and their average ad position. Spy Fu monitors 4.5 million domains.
Another good tool is the Internet Archive. You can track there how many times your competition changes their web site and is it split tested.
Finally, you might want to know who owns one of your competitors site and where it is hosted. Use Whois Source for this. Could be effective to establish partnerships as well and get in touch with the web master.
I'm still looking for a solid blog competition research beyond Google new blog search. Let me know if you found a good one.