Is there a website where I can view the wealth distribution of BTC and different altcoins?
I know such data are only estimates (address != holder, exchange wallets, etc.), but would still like to get a feeling of how different coins are distributed among its holders.
You mean something like a gini coefficient with coins?
The closest thing that you will get to will be the rich list of addresses and so on. This is the only data that is available publicly. Whether or not these addresses represent actual holders of coins will be a completely different topic, although there are products such as walletexplorer that attempts to clarify who is actually behind addresses.
So yeah, it's difficult, and I think that thus far no one has a foolproof way to quantify distribution equality.