Can I charge multiple devices from one transformer?
IF you have multiple devices that take 5V then, in theory, one supply with a higher current rating would let you charge multiple devices in parallel.
Many mobile phones, for instance, now use a standard mini (or micro) USB connection. They look for a charger with the D+ and D- pins shorted, or with a maximum of 200 ohms across them. Apple, being different, looks for specific voltages on these pins. Look up Adafruit for more details on this.
If you need other, or multiple, voltages - things get tricky. "Dangerous Prototypes" have articles on converting an old PC power supply into a voltage source. If you need 12V this may be the way to go.
Finally, a switch-mode supply is better than a transformer. Never mind the efficiency - if you want this for travel you could end up in a foreign country, with different mains voltages, frequency... and power point. With an SMPS you just need a powerpoint adaptor.
EDIT- For follow up questions - transformers have losses related to frquency of operation. The lower the frequency, the biiger the transformer needs to be. Their output is also proportional to their input, divided by the ratio of turns on primary &amp; secondary. So, a tranformer rated for 240V 50Hz ac, giving 5V out, would be ok running on 115V 60Hz - but it would give less than 2.5V out. A 60Hz core running on 50Hz mains would have larger eddy currents in the core, and risks overheating.
Simplifying GREATLY - a SMPS takes your input volts, and converts straight to DC. It then "chops" that at a very high rate, and shoves the result into a much smaller transformer. The output is monitored, and the pulse width of the "chop up" section is adjusted to get your desired voltage. Different voltage, from foreign country? Meh - the circuit just changes pulse width to compensate.
YES you can have multiple output voltages. However, feedback is typically only on one of them. Computer suppplies [see above] typically have +12V and +5V. Some have -12V, some even have +3.3V but that's not so common in PC supplies [yet].
Details of the ATX breakout board are at: http://dangerousprototypes.com/category/atx-bb/