I do not necessarily agree with or use all of these, but use this as a reference with some guidelines.
Don’t include them ‘just for the sake of it’, but if they are relevant – add them.

HIGHLY IMPORTANT ONES

Title

The Title Tag should contain no more than 70 characters (although 100 characters may be indexed depending on the engine).

<title>TitleTitleTitle</title>

Description

OK, its not exactly a META Tag, but the Description Tag can have up to 150 characters (although, 200 – 250 characters may be indexed and only a smaller portion of this amount be displayed). Also used in the browser top bar, tabs, and in the taskbar.

<meta NAME="Description" CONTENT="DescriptionDescription">

Keywords

In the Keyword tag use specific descriptive ‘words’ or ‘phrases’ to match the content on your site. You can use up to 200 characters with each word (or phrase) separated with a comma(,).

<meta NAME="Keywords" CONTENT="KeywordsKeywords">

MEDIUM IMPORTANCE – USE IF REQUIRED

Robots

The values ALL and NONE set all directives on or off: [ALL=INDEX,FOLLOW] and [NONE=NOINDEX,NOFOLLOW].

<meta NAME="Robots" CONTENT="INDEX,FOLLOW">

MS Tags

Do you want Microsoft products to automatically generate smart tags on your web pages. No? Then add this!

<meta NAME="MSSmartTagsPreventParsing" CONTENT="TRUE">

IE6 Image Toolbar

I found this one to prevent the image options toolbar popping up in IE6 over the images.

<meta http-equiv="imagetoolbar" CONTENT="no">

Expires

Use “never” unless your site will expire. (Eg. Tue, 18 Apr 2006 14:57:09 GMT | Note: Requires RFC1123 date format.

<meta HTTP-EQUIV="Expires" CONTENT="ExpiresExpires">

Cache Control

Cache control level. None / Public / Private / no-Cache / no-Store

<meta HTTP-EQUIV="CACHE-CONTROL" CONTENT="PUBLIC">

No Cache

This directive indicates cached information should not be used and instead requests should be forwarded to the origin server.

<meta HTTP-EQUIV="PRAGMA" CONTENT="NO-CACHE">

Copyright

Who is the Owner of the site? or the Company Name

<meta NAME="Copyright" CONTENT="CopyrightCopyright">

Designer

The webmasters name

<meta NAME="Designer" CONTENT="DesignerDesigner">

Revisit-After

Tell the search engines how often this page updates. (Eg. 21 days | Note: Most search engines do not support this Meta Tag)

<meta NAME="Revisit-After" CONTENT="Revisit-AfterRevisit-After">

LOW IMPORTANCE

Publisher

Owner, Webmaster, Company Name, etc..

<meta NAME="Publisher" CONTENT="PublisherPublisher">

Author

The Author Tag is for the person who wrote the material for the site. Usually the client.

<meta NAME="author" CONTENT="AuthorAuthor">

Subject

The Subject Tag is for what your site is about. Business, music, hobby, cars. You can use up to 100 characters.

<meta NAME="subject" CONTENT="SubjectSubject">

Classification

The Classification tag is similar to description but more in detail.

<meta NAME="Classification" CONTENT="ClassificationClassification">

Geography

Where are you located? Insert your full address?

<meta NAME="Geography" CONTENT="GeographyGeography">

Language

Is your site in English, Spanish, French…..?

<meta NAME="Language" CONTENT="LanguageLanguage">

City

Your City

<meta NAME="city" CONTENT="CityCity";>

Country

Your Country

<meta NAME="country" CONTENT="CountryCountry">

Distribution

Use [Global] unless it is a [Local] only site.

<meta NAME="distribution" CONTENT="Global">

SPECIAL USE

Redirect / Refresh

Used to redirect browsers to another page, or refresh the current page.
’0′ is the number of milliseconds, so use ’3000′ for a 3 second redirect.

<http-EQUIV="refresh" content="0; url=homepage.htm" >