You can watch the addresses associated with an xpub key by using blockchain.info. Either by using blockchain.info's search function, or by directly accessing the link like so: blockchain.info/xpub/xpubYOURPUBLICKEY
Alternatively you can create a "watch only" wallet by importing the xpub key to Mycelium on mobile. That would be pretty much the same solution as Electrum mentioned above, just on mobile instead of desktop.
You can create a "watch only" wallet with electrum with the xpubkey. If you truly wanted to use a website, you can enter the xpubkey in the search box and it will display all the transactions associated with that key.
Thank you for repeating exactly what I posted in your own words. I am glad to see the signature spam has made its way into the technical portion of the forum. /s