Alega
Bingo
Celeste
Corporate
Compatil
Eureka
iD:00
Nexus
Page
Point
Press
Primo
Quadraat
Rotis
Seria
Underton
Zetta
Zine

Since Stone Sans + Serif (late '80s), foundries have often launched related sans and serif designs under one name. But at any point, before or after, there are related faces with different names. For instance, from the 1930s, Gill Sans is Perpetua Sans, and Beton is Futura with serifs.

Gerard Unger produced a series of typefaces for Linotype, all with a similar skeleton but different treatments. Demos and Praxis most obviously the sans/serif "contrasting pair".

Clearface was the first typeface to be marketed with both sans and serif under the same name, in the 1910s.

Following the lead of FontFonts Scala, Eureka, Quadraat, etc., I did a sans version of Fontesque.
It gives one a flying start.

Would it have been easier to begin with the sans, and derive the serif from that? Does it cramp one's style to be working on both sans and serif at the same time? interesting questions.

Generally, when a layout uses the sans + serif versions of the same typeface, I find it's a bit flat --- not sufficient contrast. But I can see how using the styles for branding might work.

Do you know of any examples Rob, or is this a fresh idea?

But really, sans + serif is no big deal: Panoptica has seven variants...

Legacy

Palatino and Optima?

Charlotte
http://www.myfonts.com/fonts/linotype/charlotte/
http://www.myfonts.com/fonts/linotype/charlotte-sans/

Do slabs count?
http://www.fontbureau.com/fonts/Stainless
http://www.fontbureau.com/fonts/Dispatch

I was going to cite Joshua Darden's newly released Freight, but there seem to be some legal issues surrounding it, judging by this link from Garagefonts:
http://www.philsfonts.com/update.html

Serifa is Univers with serifs.

Lucida, which also has more than just sans/serif variations.

Placebo

Freight
http://www.garagefonts.com/main_new/nr_main.php

Alfon

Erik Spiekermann

"Gerard Unger produced a series of typefaces for Linotype, all with a similar skeleton but different treatments. Demos and Praxis most obviously the sans/serif "contrasting pair". "

You miss perhaps ITC Flora originally designed as a sort of Italic counterpart of them...

I have also a big family with 5 variations, who take his name from a major french newspaper... Well for fun I don't say the name and either any links :P

Freight is interesting to me. Looks like a wonderful family, possibly due for 'adoption' into the fold. Any of you agree? Disagree? Also seems that Freight is in some holding pattern due to a legality issue. ??

I'm with you Darrel. I'd be interested in Freight... or 'Litigation'...

Just got this message from Harold at Phil's Fonts regarding Freight's availability:

"Hi Dylan,

It's going to be a while but we'll let you know.

Thanks for the inquiry,
Harold Baldus"

I hate waiting for typographic treats.

Bistream Vera is also available in Sans and Serif, though there is no slanted/italic variant for Serif.

Minerva and Cimiez.

I saw Joshua's Freight in action at ATypI Prague this year and can say that it is one amazing family that will, no doubt, be used extensively in the very near future.