While a bit finicky to set up, sloper antennas can be a popular way of covering the low bands if you already have a tower for the upper HF bands.

• The 160-Meter Sloper System at K3LR 
  QST August 1994, pp. 36-38
  A quick review of the theoretical design, the construction, testing and operation of a 160-meter sloper on a 190 foot tower.
• A Reduced-Size Half Sloper For 160 Meters 
  QST March 1998, pp. 54-56
  Here's a limited-space antenna that you can put on your 40 foot tower or even a tree.
• A Two-Band Half-Sloper Antenna 
  QST June 1980, pp. 32-35
  A trapped 40- and 80-meter sloper for your 40 foot tower.