Here are the most common PSK31 frequencies:

1.838
3.580
7.035 for region 1 and region 3, and 7080.15 for region 2 *
10.142
14.070
18.100
21.080 (although most activity can be found 10 kHz lower)
24.920
28.120

* The 7 MHz band is much wider in region 2 (the Americas), thus 7.080 is added for that region.