I've been using Panic's Coda as my default editor/IDE for some time now. I've already shared the Drupal plugin I wrote on this blog. But one of the other tricks Coda allows me to do is snippet management using its Clips feature. There's quite a few Coda clips available on the net. Here I am sharing the Clips I use the most during Drupal development and theming.
A 'snippet' is "a programming term for a small region of re-usable source code or text", says Wikipedia. Of course there are a lot of snippets available for such a popular environment as Drupal.
In the Drupal handbook, the official documentation for Drupal, you can find two sections with snippets. One is for general PHP, SQL, view, block, ... snippets. The other is for theme snippets.
There are also two unofficial resources available, DrupalBin, a pastebin website for Drupal, and Drupal Snippets. The second one has few snippets available right now though. So the most dynamic resource to keep an eye on is DrupalBin.
There's probably a few more. I invite you to submit those in the comments.