A Q&D Multiband Antenna by N7FC
A brick, some PVC tubing and a few other common hardware store items can be transformed into an effective Quick & Dirty RF-launcher—for less than $25!
http://www.arrl.org/files/file/Technology/tis/info/pdf/0406058.pdf

Active Antenna - PAØRDT - Version 1 by VK6YSF
The active antenna designed by PA0RDT may offer a practical technique for exploring radio frequencies below 500kHz and is reported to offer good performance in the lower frequency bands from 10kHz to at least 10MHz.
http://vk6ysf.com/active_antenna_v1.htm

Active Antenna - PAØRDT - Version 2 by VK6YSF
The active antenna designed by PA0RDT may offer a practical technique for exploring radio frequencies below 500kHz and is reported to offer good performance in the lower frequency bands from 10kHz to at least 10MHz.
http://vk6ysf.com/active_antenna_v2.htm

HF Receive Antenna by KØEMT
If you are only using the stock antenna that came with your SWL radio, Yaesu VX-7 or Kenwood TH-F6/TH-F7 you are missing out on a lot. Give this simple parts box antenna and matching unit a try and you'll be surprised.
http://www.dbbear.com/k0emt/projant/hf_rcv/index.html

Mini-Whip Antenne by PAØRDT
After several fruitless attempts to make an active loop work in a city environment, it was found that the electric field from local noise sources was contained within the house. The magnetic field of local noise sources was not contained inside the house, making weak signal reception at LF impossible
http://www.kiwisdr.com/docs/pa0rdt_whip.pdf

Simple and better circuit for MiniWhip antennas by PA3FWM
The original PA0RDT MiniWhip is a very simple and effective active receive antenna for the longwave, medium wave and shortwave range. On this page, I introduce an alternative circuit for this application which is simpler, yet has good (probably better) intermodulation performance.
http://pa3fwm.nl/projects/miniwhip/

NB6Zep Antenna by NB6Z
The NB6Zep antenna is a simple inexpensive wire antenna, easy to build, that will yield excellent results on all bands. Only a dipole would be easier to construct and maintain.
http://myplace.frontier.com/~nb6z/nb6zep.htm

The "SQUARE"-Pole Antenna by WB2VUO
The "SQUARE"-Pole is a 1/2-wave dipole that has been bent into a square. This is the principle of the `Squalo', and the halo is the same idea, but the dipole is bent into a circle.
http://www.zerobeat.net/g3ycc/squalo.htm

The Mark II Crossfield Antenna by G3YCC
The mark 2 Crossfield Antenna was described in Sprat 76 by Alec, GD3HQR . He notes that although it showed no directivity at HF, it was possible to null out time base noise from a TV in an adjacent room.
http://www.zerobeat.net/g3ycc/xfield.htm