What is the lowest latitude at which snow has been recorded at sea level?
According to the USA Today article Answers archive: Winter, snow, ice, the sea level snow closest to the equator in recorded history are as follows.

The record for either hemisphere was in 1654, in the Chinese city of Nanning (now near present day Beihai) recorded snow, it is situated at 21°29'N.  This cold period is included in the article Coherence of climatic reconstruction from historical
documents in China by different studies (Ge et al. 2008) as part of a historical document that stated:


  ‘The winter in the 11th year of Shunzhi (1654 A.D.) was
  severely cold


For reference, the USA Today article states that the Southern Hemisphere record was considerably further from the equator, occurring in Valparaiso, Chile (33°02'S) in 1977. 

One major caveat to this answer (asides from the USAToday source) is the length of time that records have been kept - for China, it has been thousands of years - there is virtually no equivalent record keeping in many areas (particularly in the Southern Hemisphere) for that length of time.  

Having said that, according to the Australian Bureau of Meteorology, snow has been recorded in the 1950's and 1960's, several hundred kilometres north of Brisbane, the furthest north was in the Clark Ranges, west of Mackay at a latitude of about 21°S- but these locations were considerably higher than sea level, so do not qualify.

From personal experience, I recall when I lived in the southern suburbs (at sea level) of Melbourne as a child, one winter there was snow - melted as it hit the ground - but that is at a latitude of 38.2°S

In geological history, it could be argued that snow may have preceded the glaciation at the equator during Snowball Earth events, if proven to be true.