I'm doing this in the hope that it will be helpful to someone.
I personally prefer the second sentence to the first one.
Hope is a feeling of expectation and will expresses the emergence of a future event. I'm doing something and I'm expecting it to be helpful to someone in the future.
I'm doing this in the hope that it is helpful to someone.
Of course, it is perfectly fine to express it this way too.
The word is indicates that what I'm doing has a consequence to someone in the presence and I'm uncertain if it is helpful so I'm in the hope that it is.