If you want to watch niconico videos without making an account:>Manually go to the embedded video yourself
Just replace "www.nicovideo.jp
" with "embed.nicovideo.jp
" in the URL.
They don't exactly put much effort into preventing you from watching videos.>Websites that'll do that for youhttp://www.nicozon.nethttp://www.mmcafe.com/nico.html>Userscript (requires greasemonkey or tampermonkey) that'll do that for you without even having to leave the pagehttp://1mu.info/tools/niconico4guest.html
The link to the script is at the bottom. I can personally confirm that the script works, and that I had it installed for so long that I completely forgot that you normally need to log in to see videos.
All of the options work by getting the embedded video, which doesn't require a login to see, so they'll probably fail if the uploader disables that. I've never actually seen that happen though.
Anything at seiga.nicovideo.jp
is an image, not a video, and you'll still need an account to see those.