I've created a simple Google Chrome extension today called "Hacker News Collapse" that collapses comment trees on Hacker News, like the way it works on Reddit. While I created it to satisfy my own needs, I figured other people might find it useful as well so that's why I post it here.
On the comments page it appends a [-] button to the right of each comment header. Click on that to collapse the comment and all its children. Click again to reveal them again.
Chrome Extensions Galley: https://chrome.google.com/extensions/detail/bbkfcamiocfccgmcjngdljolljhifdph
BitBucket: https://bitbucket.org/drjokepu/hncollapse