For a corporate web application, I was looking into using an open source DLL that was under the Affero General Public License. Initially, I thought it’d be free because it was open source. In this case, it turned out it’s only free for commercial use if you publish your own source code. Otherwise, you have to pay for a commercial license. This might be ok in the right situation, but I learned from this not to just assume that open source libraries are free in all cases. Check those licenses carefully before you do anything!