I don't know how popular this game has been, but pewpew is basically an excellent geometry wars clone. It has really good controls, unlike many of the other two stick shooters, and it's free. I would recommend it to anyone.
http://creators.xna.com/en-US/education/gettingstarted/bgintro/Chapter3
http://creators.xna.com/en-US/create_detail
Once you run the program that you installed on your xbox, it will show a code that you have to enter on your computer through my computer, xna studios, xna device center. Then...
This is back again.
Same link, https://www.dreambuildplay.com/main/register.aspx
I think it said you have to sign up by January 18, but it's another free year!
Thinking back, I would have to go with the game that began it all: Pong. Think about it: every game you play today has roots in this simple, simple game. Pong showed people the future of video games, and has been the driving force behind decades of entertainment.
and for good measure:
J3K4B-JJH9W-6P4GC-JQFBT-TQ22Y
TWGM3-HXR7Y-4KJMT-7T9HX-3YC4M
QXT7J-GPRRW-MF9RK-CBFJK-V8KGW
BXC2B-9FQQJ-Q4MGJ-HCF9D-BPVRW
W2KMJ-GQ9C7-8WRB3-FJCGX-BR8QT
J7Q3F-WR2JY-G67YT-PXWVT-DGHWQ
HTY6X-CTMD6-QHY9K-J8R69-Q3Q9M 240...
i have no idea if all of these work, but use one if you want:
TTW3R-QCMJH-Y729H-MKXPG-CHGPG
BD6D4-PFC2F-7479G-C4MHQ-3PXGY
JP2MP-4MV3V-VBY83-BQC74-9X7GB
XCWC3-HQTXF-TFKQ4-DCDD6-DB4HB
MGPKT-JCYF9-F66MQ-3DHB9-V37CD 150...