Southwest is offering fares to/from Long Island from everywhere they fly in the Continental US for $29 to $99 each way for travel August 16-January 11. Unlike the airline's larger nationwide sale, which offers the lowest fares only on Tue/Wed/Sat, you can find the lowest seats for $99 on other days as well. This sale ends on July 26.
FYI, from Islip Southwest flies nonstop to Florida and Las Vegas; connecting flights to most other destinations. And as of this time (this was posted 11.30 AM on July 10) other airlines were not matching, but they may do so in the future.