What sites or software tools do you use to track your competitors?

I'm interested in tracking many different aspects:

• Prices
• New / Updated Products
• New features
• Negative / Positive feedback
• Press releases
• Balance sheets
• etc.

• Compete, Alexa, OpenSiteExplorer
• Google alerts

You can use Twitter to track everything you listed in your question, except for balance sheets. Check out 7 Sneaky Ways to Use Twitter to Spy on Your Competition (from the KISSmetrics blog).

I posted the below response to this question that may also help you:

• Follow your competitors. Find out what they are saying and who they are saying it to. If you don't want your competitors to know that you are following them, you can create a private list that is only visible to you. Put all of your competitors on that list and now you only have to look in one place to see the latest conversations your competitors are having.
• Search Twitter. Search on Twitter for the names of your competitors and the names of their products. See who is talking about them, who is happy with them, who is not happy with them, who has questions that you might be able to answer, etc.

Also, there is Social Mention (http://socialmention.com/)

It's not always 100% accurate but it displays a few interesting things such as positive mentions vs. negative mentions, etc.

• Compete and Alexa to track the number of visitors
• Read the forums to see feedback
• If they're a public company, you can learn a whole lot from their filings.

Google and Twitter primarily. I also read their blog and UserVoice/GetSatisfaction site if they have one.

Bets place to check out the accurate number of users visiting your competition's website is Google adplanner that has access to Google Analytics.
And for companies regarding funding and so on, go for the crunchbase powered by Techcrunch.
About alerts on what they are doing shorttime go for Google alerts!

Hope it helps!

I use RSS feeds in Google Reader. Including Twitter queries. Blog searches, Google Alerts primarily.