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 door PAØRDT
Omdat het me maar niet lukte fatsoenlijke signalen op de 160m band (op een wereldontvanger met sprietantenne ☺ ) te ontvangen ben ik op zoek gegaan naar een eenvoudige actieve ontvangst antenne.
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/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