I made this blog post to help a local school identify if they have a powerful enough power supply for their pi. However as I have had similar problems with crashing , it may be useful to others.
In my case I was having issues with Minecraft on a model B+, It kept crashing or hanging. It was suggested by Gordon Henderson I checked the power supply output.
Turns out that both the Current (Amps) and Voltage out put were lower than recommended. All power supplies have a label that gives information on Input and output levels.
In this case the output voltage is 4.7 v with an output current of 0.55A which is below what is recommended.
I tried a different power supply with better output.
In this case the output is 5v and 0.7A (700mA) which is closer to the lower recommendation level.
The notes I received with my newer Raspberry Pi (B+) suggested
Model B =5v at 700mA- 1200mA (1.2 Amps)
Model b+ 500mA to 2000m A ( 2 Amps)
I think the model A is OK with 700mA max as it has less components to power (e.g no Ethernet)If you experience problems it is worth looking at the output of the power supply unit you are using.
Today I had the following issues with my old PSU (see above)
Scratch + GPIO + pi-liter board was not working properly, with wifi connected, disconnected wifi dongle and it seemed to work a lot better.
I was also having issues getting an IP address over wifi, again this seems to be related to the amount of power supplied to the Pi,
Switching to a 5V 2Amp supply seems to have solved these issues, wifi picks up and allocates a IP address with no problem, and the Scratch GPIO works well too.
All this was working fine earlier today at the pi jam using a PSU from the Library (it was already plugged in so just needed to connect my pi in place of the library pi)
Modmypi sell a 5v 2A power supply for 4.99 inc Vat, Visit the Modmypi website for details.
Conclusion it seems you do actually need a 2A power supply. You can get away with lower but you get performance issues when you try and do too much.