Pronet Advertising has a great list of 10 Things You Should Be Monitoring online.  Other bloggers have jumped in with numbers 11–17 and 18–23.  The first ten:

1. Company name
2. Company URL
3. Public facing figures
4. Product names
5. Product URLs
6. The industry “hang outs”
7. Employee activity/blogs
8. Conversations
9. Brand image
10. Competitors

Good advice, but I’d take it a bit further.  You should absolutely be monitoring these things for all your clients, too.