The 717 was McDonnell Douglas's last aircraft design, originally designated the MD-95. It is essentially the size of a DC9-30, and was intended to replace many aging 100-seat aircraft (such as the DC-9, the old 737-200, and a handful of other, less popular types in that size range). After Boeing took over MD, they decided to rebrand the plane as the 717 (initially marketing it as a "regional jet" as it was in the midst of the RJ craze of the late 90s). However, the plane was too big to really be competitive with the RJs (the current crop of 100-seat RJs are much less expensive to operate) and too small to be competitive with the larger planes, and competed too much with Boeing's own 737.
The plane was initially ordered by ValuJet, which became AirTran in the late 1990s. Only a handful of 717s were ever built, and AirTran probably operates most of them. A few were operated by Midwest Airlines (now defunct), which were then operated by a division of Mexicana (now defunct). A few can be found in Australia. Hawaiian Airlines flies a bunch. The plane only had a production run of about 4-5 years, give or take.