I mean, for what I remember of my english classes it should be WordPress', or not?
Would this be a Meta Question?
Or a English Language & Usage one?
[update]
What I ended up writing (in an email) before Comments and was
…deal with a couple of WordPress’ projects…
[update]
Well, it ended up being an Engrish Fail.
Thanks, Johannes, for opening the question at English Language & Usage.
Toscho’s answer also prompted me to open another at Spanish Language and Usage, but for a person’s name. (oh, my, is this apostrophe correct?)
In spanish, it would be “Los WordPress”.
WordPress is a singular without plural. A second instance would be a fork that could not use the same name, because the name WordPress is a trademark of the WordPress Foundation.
Like Jesus, just more rules.
If WordPress had a plural it would be WordPresses, like in mess or mistress. But the prerequisite for that would be that WordPress becomes either a physical object or self-aware (and able to create self-aware copies). I will update my answer on the day that happens.