URL encoding the space character: + or %20?

To summarize the (somewhat conflicting) answers here, I think it can be boiled down to:

| standard      | +   | %20 |
|---------------+-----+-----|
| URL           | no  | yes |
| query string  | yes | yes |
| form params   | yes | no  |
| mailto query  | no  | yes |

So historically I think what happened is:

1. The RFC specified a pretty clear standard about the form of URLs and how they are encoded. In this context the query is just a "string", there is no specification how key/value pairs should be encoded
2. The HTTP guys put out a standard of how key/value pairs are to be encoded in form params, and borrowed from the URL encoding standard, except that spaces should be encoded as +.
3. The web guys said: cool we have a way to encode key/value pairs let's put that into the URL query string

Result: We end up with two different ways how to encode spaces in a URL depending on which part you're talking about. But it doesn't even violate the URL standard. From URL perspective the "query" is just a blackbox. If you want to use other encodings there besides percent encoding: knock yourself out.

But as the email example shows it can be problematic to borrow from the form-params implementation for an URL query string. So ultimately using %20 is safer but there might not be out of the box library support for it.