Common modem speeds nowadays include, but are not limited to:
Note the last one: that's 56k, not 56k6. People expect that extra 6 to be there to match the structure of the other common speeds. Also, 57k6 (note the 7) is one of the standard speeds for RS232 serial communications, but not for modems.
A great site about 56k, explaining some of the technology behind it.