Some authors do a really great job by collecting errors and comments to their books and putting a list on their websites. I wonder if there is some (perhaps wiki-style) website where errata are collected. Does anybody know?
There isn't. This was proposed a few weeks back on the algebraic-topology mailing list. In response to that, a forum was started to discuss this idea, called the r-forum. Contributions are welcome.
Not quite what you asked for...but useful anyway: Mathematical Errata
I think not only for books, but for published papers etc. also there should be errata lists----these could really save many a hair-pulling moment! Moreover, this will make the errata-fixing process public, and because of that probably faster and more transparent.
In fact, it would be great if such a database were created on the stackexchange.com framework, because that works so admirably for MO.
I created a proposal on stackexchange.com, if you are interested please follow it here:
Additionally, if someone has a better proposal, I am totally willing to support that too.
This very recently opened area 51 proposal here at StackExchange could fill this need:
It has a broader scope than the 2010 proposal by Suvrit, which didn't make it, perhaps this one will.
Recently, I found Math Book Notes Wiki on wikia network. There still not yet many books, but it seems promising, since it is a public wiki.
According to its front page,
The purpose of this wiki is to collect clarifications and errata of math texts. People who are learning a subject for the first time have a different perspective on the material than experts who know the whole subject by heart, and consequently are in a unique position to help each other out. If you've got any books around with the notes you made while you were learning from them, please by all means contribute! The more embarassing the misconceptions, the better -- these are the things that people don't warn each other about, and consequently cause beginning students a massive amount of grief.