5.56x45mm (AR-15 receiver)
Basically any cartridge either based off the 5.56mm, or within its cartridge overall length of 2.250". Apart from the barrel being chambered for a different round, the only other parts that may be different would be the bolt and or extractor, depending on the case head diameter . Some cartridges that come to mind are the .17 rem, .204 ruger, .223 rem, 6.5 grendel, 6.8 spc, the .300/221 variants, the .450 bushmaster, .458 socom, .50 beowulf and others. The one thing they all have in common is that their maximum cartridge overall length (COL) is no larger than 2.250".
7.62x51mm (AR-10™ and similar sized receivers)
This receiver has a magwell which accomodates cartridges with a COL of 2.81". The boltface is for the .308 headstamp and basically you can chamber for just about any cartridge based off the .308 win case or falls within the length constraints of the magwell. Some alternate choices are .243 winchester, 7mm-08, .260 remington, .338 federal, as well as some of the .325 WSM wildcats. I think the .284 winchester would be an interesting choice, with bullet choices ranging from 100gr-175gr and velocities ranging from 2900-3400fps at the muzzle for the 100gr bullets and velocities of 2100-2600fps at the muzzle with the 175gr bullets.