Thank you Halftux , interesting list!

I would like to add

Vodafone
supl.vodafone.com
PORT=7275

Thats the one I use now (switched from supl.nokia.com). And it gives me a quite quick fix.

Still most servers from the list are not accessable (from Germany) and some not even have a corresponding IP address in DNS.

Back in 2013 supl.google.com and supl.sonyericsson.com were responding with different data than supl.nokia.com (and supl.vodafone.com) so that N900 could not work with it.
So I see nokia and vodafone as the only options.
And I expect Nokia to refresh their certificate soon, as the server is not only used by N900s, but quite a lot of other devices.

And peterleinchen was looking for the certificate for supl.sonyericsson.com:
Head over to http://pastebin.com/829KdpUs
and search for "C=SE, L=Lund, O=Sony Ericsson Mobile Communications AB, OU=Sony Ericsson Secure E2E".
Next lines from (including) Begin to End Certificate should be copied into a pem-file. Thats the certificate for the issuer of the supl.sonyericsson.com certificate.

Cheers, Ulle