Does a sentence like this need a comma where the question mark stands?
Sunshine is an app for smartphones, tablets and computers [?] that lets you track and post weather news to blogs, social networks and other services.
I disagree with @Irene. The comma in question would be totally misplaced. For the sake of the English language, do not put it there!
The comma in the text would imply the sentence preceding it is self-sufficient, which it isn't. Without the second part the first part sounds awkward. The rest of the sentence simply is required for the whole to work, therefore you cannot split it with a comma.
Here the part divided by the commas isn't really needed for the whole to work, therefore you can erase it and you end up with a perfect sentence, like this:
But what if I need to further specify the subject in question? Consider:
I cannot place any commas in this sentence, as there is no "secondary part" which can be removed from the whole to work.
Now let's get back to your sentence:
If you write it with a comma, like this:
You are implying the rest can be removed for the beginning to work by itself:
Okay, that is a grammatical sentence, but how does it work? What does it say? How does it help anyone? It doesn't! It's a ridiculous sentence. So what do you do? Remove the comma!
Now that is an informative piece of text you need to present to your audience!
A comma would be inappropriate for the very simple reason that, as it's been said already, the context clearly indicates that 'that' refers to Sunshine – by means of the verb form 'lets'.
(Nobody would ever write:
but always and only:
If, for some (strange) reason, one thinks there's still room for doubt, they should rephrase the sentence. Some examples (the first that came to my mind):
Let's use now 'these softwares' (plural) instead of 'Sunshine' (singular) as the subject.
Here the reader could feel confused as to whether the subject is 'these softwares' or 'smartphones, tablets and computers', or even just 'computers'. In this case, again, a comma before 'that' would be of no help; on the contrary, it would make the whole sentence more awkward. The only solution here is a rephrasing of the whole thing.
One last observation about the sentence:
In this case, to avoid any possible misunderstanding, we just need to modify the word order:
Yes, it does. But you need to change that into which, as that cannot be preceded by a comma. So your sentence will be: Sunshine is an app for smartphones, tablets and computers, which lets you track and post weather news to blogs, social networks and other services.
This phrasing is a bit awkward, however. With a few more changes it will become more natural:
Sunshine, an app for smartphones, tablets and computers, lets you track and post weather news to blogs, social networks and other services.