There is a lot of open source engines and software with different licences conditions. Most of them are non-free for commercial use.
The question is what does "commercial use" exactly mean?
Example: I have site where I propose paid service or software. I want to create blog or wiki section on it. I download appropriated open source projects and deploy on my web site. Is it "commersial use" as I will not earn money on these engines but on the services I propose?
This question is related to
open-source
licensing
I suggest this discriminative question:
Is the open-source tool necessary in your process of making money?
If the usage of something is part of the process of you making money, then it's generally considered a commercial use. If the purpose of the site is to, through some means or another, directly or indirectly, make you money, then it's probably commercial use.
If, on the other hand, something is merely incidental (not part of the process of production/working, but instead simply tacked on to the side), there are potential grounds for it not to be considered commercial use.
Fundamentally if you use it as part of a business then its commercial use - so its not a matter of whether the tools are directly generating income or not rather one of if they are being used in support of income generation directly or indirectly.
To take your specific example, if the purpose of the site is to sell or promote your paid services/product then its a commercial enterprise.
Source: Stackoverflow.com