All implementations of a given hash algorithm must produce identical
output. So as far as reliability and accuracy are concerned there's really no issue. A hashing app is either correct all the time or never correct at all. I, myself, have written a tool that hashes files and then does stuff with the hash. I suck at math, but there's no question that my
implementation of SHA1 is correct since it produces verifiably correct results; the same is true for everyone else's implementation no matter how they implemented it.
Ease of use and special features are the differentiating factor for checksum calculators (in addition to computational speed, memory requirements, etc. but these only really enter into matters when working with large files (>100MB.)
I think that checksum apps are perfect for the context (right-click) menu in any file manager. My favorite is MD5 Context Menu
and it does just what it says on the tin; no bells and whistles to get in the way. This tool, sadly, only works on 32 bit versions of Windows.
Another good one, which has replaced MD5 Context Menu for me under 64 bit Windows is HashTab
. This one adds a tab to each file's Properties dialog window from which up to 15 different hashing algorithms can be run (including MD5 and SHA1) at once.
Both MD5 Context Menu and HashTab are free and respectably fast, and HashTab also has a Mac OS version. MD5 Context Menu isn't actively supported by the author anymore.