Free online Greek materials: -
This site has a variety of public domain Greek and Latin grammars, vocabularies and selected readings.  All of them are in the Acrobat PDF format.  For a good selection of Greek readings for beginner to intermediate from the dialogues of Plato see
Easy Selections From Plato, Arthur Sidgwick 

The Perseus Project - This is a great online Greek study resource for Plato and other classic Greek texts.  All words in the Greek text are linked to several Greek lexicons.

Berkley Greek Tutorials - A pronunciation guide with spoken examples. If you get a message to click ok to run an activeX control, do yourself a favor and just hold the enter key down until it stops.  This set of tutorials from Berkley University was prepared by the author of "Introduction to Attic Greek" below.

Build a Plato Specific Greek Vocabulary - This page tells you how to build a vocabulary list directly from the dialogues of Plato using the Perseus Project Greek vocabulary tool.  Its a good  way to make your vocabulary study time have maximum impact on Plato!

Vocabulary Cards - Classical Greek cards from VIS-ED.  1000 cards with over 3000 vocabulary words.

Pocket Oxford Classical Greek Dictionary Cheapest classical Greek dictionary out there.

Henry George Liddell, Robert Scott, An Intermediate Greek-English Lexicon This is founded on the Classic listed below.  It is for those with intermediate knowledge of Greek.

 A Greek-English Lexicon, Ninth Edition with a Revised Supplement  This is $150 and is good, but is probably overkill for beginners (the intermediate is $45)  Some prefer the abridged version below as being easier for beginning and intermediate users than the Intermediate version.

Henry George Liddell, Robert Scott,Abridged Greek-English Lexicon The shortest and cheapest of the L&S lexicons.

Greek Language Books:
Athenaze:An Introduction to Ancient Greek Book I -
This is a nice reading oriented way to learn Greek. 
There are a few books including:
1. Athenaze, An Introduction To Ancient Greek - the main text
2. Teacher's Handbook - with translations of the readings, and answers to exercises
3. The Athenaze Workbook -- drill and practice
4.  There is also volume two.
All Athenaze on Amazon

Wilding's Greek for Beginners Just enough for total beginners.

Introduction to Attic Greek This one is a good book for self teaching.