I wanted a go at writing a Safari extension, so this morning I started writing BetterHN; a little extension aimed at incrementally improving Hacker News. So far it adds two features:
1) Comment thread folding
2) Clickable links in text-based submissions
Source: http://github.com/sant0sk1/BetterHN
Install: http://jerodsanto.net/downloads/safari/BetterHN.safariextz
Enjoy!