What substance that is liquid at STP has the lowest freezing/melting point? I think it's either 2-methylpentane or 3-methylpentane, but I get different numbers from different sources.
After a bit of random searching through small molecules, I found this very large dataset with 28,645 experimentally measured melting points. Sorting them by increasing melting point and checking the first few entries individually, it seems that N,N-diethylmethylamine holds the record, with a melting point of -196°C (!) and a boiling point of 63-65°C.
I also mentioned another possibility for very low melting liquid substances at room temperature, namely ionic liquids. Even though the species present are much larger and have ionic interactions, presumably the ions pack so extremely poorly in the solid that freezing is very unfavourable. Aldrich already offers multigram quantities of ionic liquids melting at -88°C, and ongoing research offers even lower melting alternatives, such as the ones in this article which purportedly do not crystallize until -150°C. This limit may well be improved upon in the next few years.
If mixtures are allowed as well as pure substances, then a combination of small organic molecules or ionic liquids will very likely reach even lower temperatures than the examples above without freezing.