  Millisecond X-ray pulsars consist of a rapidly-spinning neutron star
accreting from a low-mass stellar companion, and are the long-sought
evolutionary progenitors of millisecond radio pulsars, as well as promising
candidate sources for gravitational radiation. The population of these sources
has grown significantly over the last three years, with the discovery of six
new examples to bring the total sample to seven. Three sources are ultracompact
binaries with H-depleted donors and orbital periods of approx. 40 min, like the
185 Hz pulsar XTE J0929-314. Three more have orbital periods of 2 hr or longer,
similar to IGR J00291+5934, first detected in outburst by INTEGRAL in December
2004. The neutron star in this 2.46 hr binary has the most rapid spin of the
accreting pulsars at 599 Hz. The most recently-discovered pulsar, HETE
J1900.1-2455 (377 Hz), has an intermediate orbital period of 83.3 min, and has
been active for more than 1 yr, much longer than the typical transient
outburst. Pulsations were detected only in the first few months of the
outburst; this source has since resembled a faint, persistent non-pulsing
low-mass X-ray binary, typical of the broader low-mass X-ray binary population.
