Standards: Clean, crisp, cold. More readable than Helvetica, i.e. suitable for body text. No tails (a, l), single story g, possibily stright y. Can even have humanist traits.
Free Universal -- ofl. Truly good for body text only at +21px (see Firefox issues).
Standard: Very legible, maximum clarity. Rational, clean, cold to some extent, with no or minimum “artistic addition”. Kinda following the Frutiger / Gill Sans distinction.
Standards: Artistic influence allowed! Calligraphic elements, organic feel. Warmer side of the Frutiger / Gill Sans distinction. Genereally, with double-story g.
Sans Guilt -- ofl, single weight
Alegreya Sans -- gfonts, brick
Rosario -- gfonts
Martel -- gfonts, 7 weight ita
Classiq -- yof
Conte -- yof
Picadilly -- fontsquirrel
Delicious -- fontsquirrel
Calluna Sans -- tumblr, free weight reg
Gandhi Sans -- fontsquirrel
Railway (ITC Johnston) -- fontsquirrel, ofl
Hammersmith One (ITC Johnston) -- gfonts, fontsquirrel
Cabin -- gfonts, fontsquirrel
Cabin Condensed -- gfonts
Quattrocento Sans -- gfonts, fontsquirrel
Molengo -- gfonts, single weight
Open Sans -- gfonts
Lato -- gfonts, brick
Surface -- fontsquirrel, single weight bold
Rambla -- gfonts, fontsquirrel
Merriweather Sans -- gfonts, fontsquirrel
Woodford Bourne -- fontsquirrel, reg and italic
Permian Sans -- fontsquirrel, reg, italic and bold
Sabon has been paired with: Helvetica / Akzidenz Grotesk, Univers, Futura, Avant Garde, Brown, Franklin Gothic, Trade Gothic and FF Bau, Nitti Grotesk, Azo Sans, FF Typestar, Century Oldstyle, Gill Sans, Clarendon and Akzidenz Grotesk, Tilde and Gza.
Minion has been paired with: San Francisco, Helvetica etc, ITC Conduit, Futura, Nitti Monospace and Romana, P22 Underground and Tablet Gothic and Georgia, Gill Sans, GT Walsheim, FF Super Grotesk, Brandon Grotesque, Avenir, Clarendon and League Gothic, Inconsolata, Alternate Gothic, Wigrum and Atlas Typewriter, Brown, Letter Gothic and Proxima Nova, FB Agency and Neuzeit and Prestige →, Courier and DTL Nobel →, Scala Sans, Antique Olive,
HELVETICA -- None. If, then, on a case by case basis, Arial, Vremena Grotesk, Source Sans Pro, Roboto
GROTESQUE -- Work Sans, Karla
APERCU -- HK Grotesk, Karla
UNIVERS -- None. Universal Free is almost unusable. If, then these are the options: Hk Grotesk, Montserrat (brick.im), Trueno (Montserrat fork), Muli, Noto Sans, Museo Sans