Ok, these are pretty basic and most people know them already.
But I’ll write them anyway, because I tend to forget everything I don’t use often. So I’ll use this as a notebook available from everywere to anyone…

link:url Shows other pages with links to that url.

related:url Same as “what’s related” on serps.

site:domain Restricts search results to the given domain.

allinurl: Shows pages with all terms in the url.

inurl: Like allinurl, but only for the next query word.

allintitle: Shows only results with terms in title.

intitle: Similar to allintitle, but only for the next word. “intitle:seoforgoogle google” finds only pages with seoforgoogle in the title, and google anywhere on the page.

cache:url Shows the Google version of the passed url.

info:url Shows a page containing links to related searches, backlinks, and pages containing the url. This is the same as typing the url into the search box.

spell: Will spell check your query and search for it.

stocks: Will lookup the search query in a stock index.

filetype: Restricts searches to that filetype. “-filetype:pdf” to remove Adobe PDF files.

maps: If you enter a street address, a link to Yahoo Maps and to MapBlast will be presented.

phone: Enter anything that looks like a phone number to have a name and address displayed. Same is true for something that looks like an address (include a name and zip code)

site:www.somesite.net “+www.somesite.+net” – Shows you how many pages of given site are indexed by google

allintext: Searches only within text of pages, but not in the links or page title
allinlinks: Searches only within links, not text or title

Some external links on topic:
Basic search help
More search help

These “instructions” are commonly used for SEO.

Hope it helps,
Vasilis Mavroudis