Personally two of my favourite shortcuts are:
- ⌃⇧L (that's ctrl+shift+L): Which wraps the currently selected text with a link to whatever's in the clipboard, and works for every text language I've tried it in.
- ⌃⇧⌘L (that's ctrl+shift+cmd+L): Which googles for the selected text and links to the top result.
The are both super useful for writing text and blogging, (and stackoverflow).
Codewise, I think that I prefer snippets to key shortcuts. Being able to type if⇥
etc., in almost any language is ridiculously useful, and the consistent interface is what keeps me using TextMate.
I also found this quite amusing. But I prefer to learn my shortcuts in small steps, and often find that just looking in the gear menu (⌃⎋) works.