My post is redundant but Ill put my vote in as well,
Everest Ultimate (proves the most accurate IME), CoreTemp is another very accurate program. I like Everest b/c of sidebar support as well as Systray/taskbar icons. I also use an Asus Utility that came with my mobo called PC Probe II. Its flexible like Everest and has been very accurate.
DTD I use Everest and when testing/benching/stressing Ill cross check with Core Temp and PC Probe II
I also have an Ultra MD3 Card reader with a USB/Firewire/eSATA port as well as fan controller and 2 temp probes. I place one between the DIMMs and the other on the HSF by the CPU, and always within +/-2*C of these programs.