I've never found plug-ins useful.
I use only one computer, so I only have one set of bookmarks, and I use MobileMe running through a mobile browser I set in Dropbox to sync everything.
Dunno why page info ever matters, really.
And I feel stupid saying this, but I've never had to debug a page. Things run more or less logically. If something messes up you just look at your code again.
Yes but so far I'm not aware of any. Not even the google toolbar AFAIKS.
For me plugins like online bookmarks (delicious), current page/site info (seoquake) and web debugging (firebug) have become 'must haves'. There are many others that I have tried and found I did't need but it is nice to have options.