To generate a QR code online for a webpage, you can check out these 10 websites as mentioned before.
And if you only want to generate a QR code for a Wikipedia article, then you can check out the QRpedia website, which is professional and easy for you to do that online, free of charge.
On the homepage of QRpedia, you just need to enter the URL of a Wikipedia article in any language, and press the Enter key, then you can get a QR code for that URL on the same page.
For the generated QR code, you can download and share it with your friends, who then can scan it with QR readers and visit the Wikipedia article on their mobile phones directly.
For example, if you enter the following Wikipedia article URL:
QRpedia will shorten the above URL with its own domain to the following one:
The above URL will be turned into a QR code (as what you can see from the image in this post), scanning which, you can visit the shorten URL and then the original Wikipedia one on your mobile phone.
By the way, QRpedia will only work for the "Read" pages of Wikipedia articles, not for image, "Talk", "Edit" or any other Wikipedia pages, even though it may generate QR codes for those pages, the URLs won’t be valid at all.