I have a Bushnell TRS-32 on my AR15 and it's doing great for $129. Designed to co-witness with irons. TRS-25 for $99 and great reviews, many folks using on AR's also. Both these from WalMart. They also have a TRS Trophy model(looks like ACOG) for $179(red/green) but don't know much about it.
Vortex Sparc has one for around $200 that has very good reviews. Burris Fastfire is around $250. Mueller has a Red Dot for around $100(?) and Mueller has a very good CS.
I also have a Bushnell Trophy(different model) on a 20ga shotgun that has 4 different reticles(red/green) for around $170 that has held up great.
Have a TRU GLO on a little 9mm carbine that has done very good for around $70.
I've also got some older Center Point Red Dots(WalMart) that did very well for around $40, and now they have some new models out that look even nicer.