My Lightroom library consists of all my Raw files as well as files I've created tiff Photoshop "projects", not to mention the thousands of images from my pre-DSLR pre-Raw days, so LR is my main browser -- waiting a few seconds for standard previews to be refreshed is no big deal to me.
Aternatively I'll sometimes go to Bridge for something, or, in recent months, DPP for browsing and sometimes for a quick in-and-out conversion of some shots.
So, the only time I would browse at anything but the full original resoution is when I'm checking into a directory that I've used to store jpeg conversions, and then any old thing will do -- I can set my system file browser to show thumbnails and I can view them in my system image viewer quickly so no pain.
When I do a Web image conversion I follow POTN rules, as in a max of 1024 at the widest dimension, although occasionally I'll veer from that for, say, a tall portrait-oriented shot that I may prefer to be 800 pixels tall. For images I want to upload from my computer, I'll size at 800 max, as wall as "unimportant" images I may upload to my Web host but are not for real gallery/image sharing use.
For email sharing, I guess something similar applies -- if it's a "good" image I may prefer the 1024 max, for others 800 max. But, I more often will point people to my Web host home page or a specific gallery of interest. For "special" shoots I do for an individual but that are not meant for "public" view I password-protect the galleries and will confide that password to the client or friend.