I'm looking at, for example, July 19. Disneyland.com shows 8am - 11pm. Disneytravelagents, mouseplanet and disneyextreme show 8am - midnite. Who knows what is right? Also for California Adventure
I would trust Disneyland.com (8-11 during the week in the middle of July) as the most reliable since it is the official Disney web site. The others may be basing their hours on historic information (last year, I think the hours were 8-12 most of the summer). Disneyland.com, although not always up-to-date on other things is usually pretty up-to-date on its hours. However, they may choose to extend the hours based upon crowds. Like most things, hours are subject to change.
PS -- Keep checking back with Disneyland.com for the latest hours.
I know that some people have called and received confimration that the hours on the DL website are indeed true. CMs have also received similar info regarding work schedules.
DL website changed the hours so I would guess they are really open until just 11pm.
I had checked earlier too so I could make a planning calendar and the hours used to be to midnight -- that's probably why all the other sites have midnight. But they are now switched to 11pm.