Core Temp (64-bit)
Full user review
"Great for status bar temp display"
Works quickly, and unobtrusively. Been using it for a few years now on different systems. Never had an issue with accuracy.
When overclocked via BIOS, CoreTemp can no longer read temps, negating any usefulness. If you never overclock, this tool should be perfect for you.
If you don't overclock, this should be in your 'must install' list for a new OS install.