http://marketplace.xbox.com/en-US/Product/Space-Shooter-Bundle/66acd000-77fe-1000-9115-d80258411418
It is 4 XBLA shoot em ups all sold together for a discount, $20 or 1600msp. It includes Sine Mora, Ikaruga, Radiant Silvergun and Fusion: Genesis.
Now, I have only read about Silvergun and haven't tried Fusion but I can definitely say that this bundle is worth it just for Sine Mora and Ikaruga.
Sine Mora is what I call a 2.5D shmup, because it uses 3d graphics but you play on a 2d plane. It is one of the best looking shmups ever, even better looking than Omega Five. It has a sort of WW2 theme to it.
Ikaruga is lauded as a classic game and also as incredibly difficult which it is. It's fun as hell, though. The mechanic of the game is that every enemy and bullet are either black/red or white/blue. Your ship is the same way and you change polarity quickly with a button press. You absorb bullets of the same colour as energy to shoot back, and you deal double damage to enemies of the opposite polarity.
Radiant Silvergun is the spiritual prequel to Ikaruga. It has similar elements and is fast-paced but does not have the polarity-switching mechanic. Again, I haven't played it but it is a celebrated classic shmup and I have only ever read good things about it.
Fusion: Genesis looks pretty and fun which is all I can say about it, I am downloading the trial now.
You can download demos/trials of all 4 of these titles from XBLA. Even though I already own Ikaruga I am seriously considering buying this, as Sine Mora is usually $10 or $15 by itself and I have been meaning to purchase it.
Sine Mora
Ikaruga
Radiant Silvergun
Fusion: Genesis