Introversion's critically lauded Darwinia has arrived on XBLA after a lengthy period in limbo. It brings originality and a very interesting visual style to the table.
It'll set you back a fairly hefty 1200 MSP, but you're getting two games in the 'box', as it were, and a serious amount of depth. You can cue up the download for your 360 by hitting the link on the right.
Let us know what you think about one of the stranger games to hit XBLA in some time.