Normally this relates to pooled mining, as already mentioned - I believe OP's asking about the lowest hash on blocks regardless of whether this was at a pool or 'solo', though.

Yes: parse the blockchain, get the hash value for each block
Kinda: parse all the blocks, including orphaned and otherwise rejected, get the hash value for each block.
No: in case a pool glitched or somebody withheld a block, etc., there's always a possibility - however remote - that the actual lowest hash is lost forever.

I suspect you were wondering if there's a site that already provides this statistic, though - I can't think of one off the top of my head, but I would guess that there has to be one.
Perhaps organofcorti already tracks this - if not, it's exactly the sort of thing that would fit right in on his site as one of those once-in-a-while posts
I did something similar for some altcoins a while back that had different PoW models, and checked if the nonce etc. had any influence on whether or not a block was found, what the hash of a block was, etc.  made for some fun graphs (had to learn graphing stuff with OO.o and later LO for work, was good exercise - of course LO goes and breaks stuff every other release... right now you can't custom-position the legend, it resets to top-left on file open. gah. /rant)

I can tell you that between blocks 328,000 and 350,999, block 334,261 with a hash of 000000000000000000002d414bb8f9175ba6c6563721e1ba2c1373c2bd94f29f appears to be the lowest - and I can only tell you that because those blocks were still in memory from an unrelated topic
There's bound to be lower further up in the chain, though - it'll just take longer to find them as the ratio of such low hash blocks vs all block hashes becomes lower.