We may [allow everyone to select their own font](/meta?q=56), but whether we do or not, we need to choose default fonts for TopAnswers/databases (both regular and monospace for `code`).
Please answer below with suggestions, one suggestion per answer, so we can vote and figure out the most popular.
The fonts must be available under a suitable license such as [OFL](https://en.wikipedia.org/wiki/SIL_Open_Font_License). A good place to look might be this [font library](https://fontlibrary.org/en/search?order=pop&license=OFL+%28SIL+Open+Font+License%29), and if you want sans-serif in particular you can [filter out the rest](https://fontlibrary.org/en/search?order=pop&license=OFL+%28SIL+Open+Font+License%29&category=sans-serif).
I would like to suggest [Source Sans Pro](https://fontlibrary.org/en/font/source-sans-pro).
| Item | Description |
| Category | Sans-serif |
| License | OFL (SIL Open Font License) |
| Family | Source Sans Pro |
| Designer | Paul D. Hunt |
| Foundry | Adobe Systems Incorporated |
: Sans serif font family from Adobe for user interface environments. http://adobe-fonts.github.io/source-sans-pro/ Roman fonts version 2.020 and Italic fonts version 1.075 (OTF, TTF, WOFF, WOFF2, EOT)
**Full Language Support**
: Afrikaans, Baltic, Basic Cyrillic, Basic Greek, Basic Latin, Catalan, Central European, Dutch, Esperanto, Euro, Igbo Onwu, Pinyin, Polytonic Greek, Romanian, Turkish, Vietnamese, Western European
If the [Bitstream Vera](https://en.wikipedia.org/wiki/Bitstream_Vera#Licensing_and_expansion) license is acceptable I would vote for the [Deja Vu Sans](https://fontlibrary.org/en/font/dejavu-sans) font.
It resembles the widely used Verdana when it comes to font width and supports many languages and symbols as well as a monospace variant for code blocks
Full Language Support
Afrikaans, Arabic, Archaic Greek Letters, Armenian, Baltic, Basic Cyrillic,
Basic Greek, Basic Latin, Catalan, Central European, Chess Symbols,
Claudian Letters, Dutch, Esperanto, Euro, Farsi, Georgian, Hebrew, Igbo Onwu,
IPA, Lao, Latin Ligatures, Mathematical Operators, Ogham,
Pan African Latin, Pinyin, Polytonic Greek, Romanian, Tifinagh,
Turkish, Uighur, Vietnamese, Western European
It also just looks nice and is already included in a lot of linux distros
I would nominate [Hack](https://sourcefoundry.org/hack/). For many of the same reasons outlined for Deja Vu Sans, but with many improvements for use in source code.
*Caveat:* This should only be used for code blocks, body text should be a Serif font as discussed elsewhere.