Since Visual Studio is my prefered development tool i've installed tons of add-ins over the past few years..
most has been raved about from one end of the world to another but i've shifted through quite a few so far but some of my favorite ones (which are always on my little trusted microsoft usb key (whooping 32mb of space)) i'm listing here below:
GhostDoc -
www.roland-weigelt.de/ghostdocCodeKeep -
www.codekeep.netSlickEdit Gadgets -
www.slickedit.com/content/view/441and one of my favorite sites, dedicated to Visual Studio is
www.visualstudiohacks.com - which is run by James Avery (who incidentially also authored a book of the same title. eg. Visual Studio Hacks, by o'Reilly).