I heard that using the crescent on a mosque is an innovation. Is it permissible to use the crescent as a logo for Islamic sites? Is this an innovation?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Using the crescent as a symbol is something which the former Muslims did. It is for this reason that you may notice this on flags of some countries like Pakistan, Turkey, Tunisia, Algeria, Mauritania, and Malaysia. It appears that this is not done as an act of worship or an act by which one gets closer to Allaah; therefore it is not an innovation. It might be that those who used it as a symbol did so because Allaah considered the crescent as a sign of starting the fast, performing Hajj, paying the debts, paying people's salaries and so forth. Allaah Says (what means): {They ask you, [O Muhammad ] about the new moons. Say, “They are signs to mark fixed periods of time for the people and for Hajj [pilgrimage]".}[Quran 2:189].

To conclude, there is no harm in beautifying the site with the crescent or taking the crescent as a logo for the site.

Allaah Knows best.