These links have been chosen to help students having difficulty with reading. They are also appropriate for English Language Learners in general.
Learning Letters and Sounds
Alphabet "Whack-a-Mole"-click the letters in alphabetical order
Spring Bee-help the bee gather the letters in order!
Learn to Read at Starfall.com-Level 1, the ABCs.
Letter Sounds-beginning consonants (3 levels)
Letter Sounds 2-Blends-beginnings like cl, cr, bl
Letter Sounds-'S' Blends-beginnings like st, sl, sp
Letter Sounds-Short Vowels-beginning sounds are short vowel sounds
Letter Sounds-Long Vowels-beginning sounds are long vowel sounds
Alphabet Zoo-match names and first letters
Phonics: Putting Sounds Together
Learn to Read at Starfall.com-Level 2. 
Reggie the Rhyming Rhino-play this rhyming word game from Scholastic
Room 108 Phonemic Processing-an excellent activity! Match letters and sounds, exchange letters to make new sounds; five levels.
Phonics Games and Activities (from Sadlier-Oxford Publishing)-Graded phonics lessons in game form. There are many activities here; the emphasis is on letter-sound relationships, but by Grade 3/4 there is more vocabulary and grammar.
Sight Words
Room 108 Word Games using the Dolch Sight Word List (220 most common words)
Dolch and Fry Sight Word Reading Games-match the cards
The Rain Game-spell the word by catching raindrops
The Sentence Game-put the words in order to make a sentence
Hangman-word sets : colours, seasons, months, days of the week, numbers
Reading Games-12 interactive games from the Mansfield/Richland County Public Library (Mansfield, Ohio)
Learning Words
Nina the Naming Newt-find names of things from many settingsManyThings.org-picture-and-word games used to teach non-english speakers. Several types of games take the learner from new word to recognition.
Learn English with Pictures and Audio-Designed for english language learners.
Learning Chocolate!-another picture-word site with audio
Improving Vocabulary
Readquarium-(from Gamequarium) Build your knowledge of words, meanings, usage. There are lots of activities here, but avoid the "Ads by Google."
Readquarium-(from Gamequarium) Build your knowledge of words, meanings, usage. There are lots of activities here, but avoid the "Ads by Google."
Gamequarium's Vocabulary page is another source; there may be some duplication of activities.
FunBrain KidsCenter-scroll down to the Words section. Again, lots of activities, but avoid the in-house advertising.