Slacker Radio for WM6(.x)
Finally! Just about every other smartphone platform has it, and it came to XDA earlier in the year via the T-Mobile HD2’s ROMs but now it’s become official, Slacker Radio has an official Windows Mobile client. Although it is listed for WM6.5 WVGA devices only, users are reporting that it works fine on WM6.1 WVGA devices as well.
(Note that you require an IP address that belongs to the USA to use this free streaming radio program.)
Thanks to acidbath5546 for the tip!