SearchMetrics.com tracks visibility for most sites, updated every thursday and has data going back to 2010. It's a paid tool, but if you want to pm me the site, I can run a quick report on it. Only probs is it doesnt track visibility for Google.ie
I've never seen that kind of thing in Webmaster Tools - so you're saying they'd basically tell you if you're penalised?
Interesting.Yes. They send the webmaster an e-mail for most issues and also show a message in the GWT when you login, as well as a warning flag.. For example, if they detect paid links, you get an e-mail "Unnatural links found on domain....." They'll also send a note if you've been manually penalised. An algorithmic drop you have to detect yourself and is fixed automatically.
So if you used to get 5000 clicks per month and now get 2500, you'd get a mail saying "Dramatic Click Through drop for [domain.xyz]"
A recent addition to Google webmaster is the ability to allow you to disavow a link to a website, so basically if you have a bad link pointing to a domain you own that is affecting your ranking, you can ask Google to ignore it. Personally I think it's taken far to long for them to do this. it also suggests negative SEO is a distinct possibility.