ISS

The International Space Station is populated by astronauts who are all Amateurs.

To downlink is 145.800 FM. (There other frequencies outside the Amateur ranges such as 143.625 FM on which you can hear ISS to base communications)

Working the astronauts

In Region 1 (Europe-Middle East-Africa-North Asia), the uplink is 145.200 FM.
In Region 2, (North and South America-Caribbean-Greenland-Australia-South Asia) it's 144.490 FM

Please be aware that there are scheduled links with schools, and please don't attempt a QSO if someone aboard the ISS appears to be answering questions.

You can apply to have a scheduled contact for a school here: http://www.ariss.org/

Working the cross-band repeater

To use the cross-band repeater, set your uplink to 437.800 FM (worldwide). However, according to "ISSFanClub" as of 2008-10-19, it hasn't been used for 255 days.

Working it

The ISS will take about 12 minutes to pass from horizon to horizon if it goes directly overhead. The signals (as received on a mobile antenna (6/2/70cms), leant backwards to "go horizontal") are 5/9+30 at the peak of the pass. It's a very strong signal, as the station is only 200-300 miles away (straight up!).
However, from that height, the ISS can "see" a large area of earth (footprint) - all with many stations wanting a contact.

Images

Occasionally, the 145.800 FM downlink will squirt some data ever 5 mins. This appears to be images.
Images can be submitted to ARISS, and are visible here: http://www.amsat.com/ARISS_SSTV/

A very good page for more information is http://www.issfanclub.com/