I copied these from a document available through the Math U See Yahoo support group. None of the ideas are mine. 
But looks like there are some good ones! Hope they are helpful to those drilling addition facts!
BASIC FACTS MEMORIZATION IDEAS
The following are ideas for ways to practice and learn basic facts, which is a portion of the Math-U-See mastery process. Many of these have been submitted by members of the mus-users Yahoo discussion group. Credit has been given to the contributor when known. Inclusion on this list is based on the suggestion of parents and users and does not constitute an endorsement by Math-U-See.
1. http://quizlet.com/ - Free site lets you make flashcards for use online or to print, or you can search to see what others have already created. Also games you can play to learn whatever is on the flashcards. ~Darlene
2. With 2 children, have the one stronger on facts go through flashcards first while the other one listens. It’s like priming the mental pump! ~Deanna
3. http://www.tutpup.com/ - Choose which fact and what difficulty level you want and then you play against other children from around the world. ~Brenda
4. Bingo - To practice sums to 10 lay out numbers 0-10 for each child and when they draw a card from the stack, if it's a 4, they turn over the 6 card, etc. Depending on the number of cards, sometimes the only way to "win" is to turn over all the cards in a horizontal line.
5. Memory - 2 sets of 0-10 and put the cards upside down (separating the sets). If they flip over a 4 in one set, then they try to find the 6 in the other set.
6. Wrap-Ups - They are low tech, low cost and portable. I used to keep them in car and tell the kids to work on certain facts while we were going someplace. ~Laura
7. Flashmaster - Sort of a like an electronic flashcard. ~Laura
8. Aggravation – Use with flashcards. If they get one correct, then they can leave base and move to the start. They also need to get one correct to move a player to the next spot so that they could get another player into the start spot, etc. Once a child's players are out of the base, if they get it correct, then they move that many spaces. ~Deanna
9. Memorize in Minutes by Alan Walker - For my daughter who is not mathematical, has very poor memory but is very visual, it works where nothing else has. ~Sarah
10. http://www.iknowthat.com/com/L3?Area=Mathblox – Similar to Tetris.
11. http://www.scribd.com/doc/7076612/MUS-Subtraction-Chant - Chant to help with subtraction (making 10) facts. ~Kellie
12. http://www.schoolhousetech.com/BasicMath/Default.aspx - Math Resource Studio 4 Lite. We use the free trial version and do a "timed worksheet" every day. ~Laure
13. http://www.mathusee.com/math_drill.php - Online drill at the Math-U-See web site.
14. http://www.citycreek.com/ - Addition the Fun Way and Times Tables the Fun Way.
15. http://www.audiomemory.com/ - Addition Songs, Subtraction Songs, Multiplication Songs, Division Songs.
16. http://www.memorydynamics.com/index.html - Arthur Bornstein’s Memory Power Memorizer Kits. ~Laura
17. Writing the facts with a finger in something with texture (shaving cream, pudding, cornmeal, rice or sand).
18. Write the facts with a marker on a beach ball. Throw the ball back and forth and recite the fact closest to a given finger. (For example, say the one closest to your left index finger.)
19. Recite the facts while doing something rhythmic (jumping rope, rocking, bouncing on a trampoline).
20. Playing hopscotch with the facts in the squares of the hopscotch board.
21. Draw a large calculator on a shower curtain or vinyl table cloth. Child jumps on the correct buttons for the equation.
22. Make troublesome facts the "password of the day" or someone's name for the day.
23. Build each fact with blocks, say it aloud and write it.
24. Math Fact War – Use standard deck of playing cards. All face cards are 10. To work on an individual set of facts (for example +4), turn over top card from deck of cards. The first person to give the correct answer (the value of the card +4) keeps the card. The person with the most cards after going through the deck or a predetermined amount of time wins. Once all facts have been covered, turn up two cards at a time and give the sum (or product) of the two for the answer.
25. Drill difficult facts immediately before going to sleep for the night to assist with transfer from short-term memory.