In my opinion I haven't found anything better than halo. A lot of people like to hate on it and claim that there are a lot of better shooters out there but the proof is in the pudding. Halo sells 8-10+ million copies on a single console for a reason. Halo 3 continues to be one of the most played xbox live games for a reason and it's because its a great game and because halo is a great series and there isn't much if anything at all like it.
If it isn't anything special then why can't developers make another sci fi shooter that's as played by as many people or loved by as many fans as halo?
Yeah so long story short you might have to pony up the money to get a 360 so you can get your halo fix because there aren't really any substitutes for it.