The Sensor Network Museumtm - Antennas
Even if antennas are rated fully weatherproof, often the joint to the coaxial connector is not. An additional silicon sealing improves the situation a bit.
2.4 GHz Antennas - Tmote Sky
Hyperlinktech external antenna HG2407RD-SM
Moteiv has documentation about this on their website: http://www.moteiv.com/community/Adding_an_External_Antenna_to_Tmote_Sky
868 MHz Antennas - Tinynode
Antenna Factor ANT-868-CW-QW Datasheet [pdf]