LM2574 and similar parts from national semiconductor do the task,but they are non-isolated,you can also buy DC/DC converters at reaonable prices from TI and minmax(www.minmax.com.tw) they have 24 to 5 at 0.5A
We have positive result with NATIONAl LM3478 + external MOSFET
with this IC you can make many others topologies.
In our case we develop -60V -> -12 V with common +60 rail.
Tha Maxim MAX724 is nice to use - internal switch and just the usual few passives and small inductor. Output voltage is adjustable, input can be up to 40V, 100KHz oscillator, 5-pin TO220 package.
MC34063 is very cheap and easy to design, it can be configured as step-up or step-down mode. But it's working freq is low, that means a big inductor is needed.