There are probably a couple hundred "browsers", though most of these use the Internet Explorer core and only provide an alternate shell.
"Unique" refers to including a rendering engine as opposed to using one already included on the system. It does not discriminate against browsers that use the same or similar rendering engine but integrate it directly into the program (such as Chrome and Safari).
Internet Explorer*
Mozilla Firefox
Apple Safari
Opera
Google Chrome / Chromium*
Seamonkey
K-Meleon*
Dillo
Arora
Netscape
Grail
W3m
Slipknot*
*currently available only for Windows
Maxthon
Avant
WebbIE
AOL Explorer
UltraBrowser
iRider
MSN Explorer
Sleipnir
Enigma
NetCaptor
Runescape "stand-alone" client
GreenBrowser
Lunascape
Runecats Explorer
Teega
Tencent Traveler
Yahoo Browser