Dual booting and having multiple ROMs on your device is something a lot of Android users have been waiting for. It can be very annoying having to reflash ROMs all the time to try out different ones. There is now an app called BootManager which will let you have multiple ROMs on your device (up to 3 ROMs). However there is a catch, the ROMs will be placed on your SD Card so they won't be as fast as if they were on your internal storage. Also not every ROM out there will work.
The app at the moment is officially supported on the original Motorola Droid, Droid Incredible, HTC Thunderbolt, HTC Vision, HTC EVO 4G, HTC Inspire 4G, HTC Desire, and the HTC Desire HD. It may work on other devices but not all have been tested fully.
Note: This is for advanced Android users.
Step 1 - Flash your second ROM
Obviously before you do anything make sure you have a nandroid backup so incase anything goes wrong you can restore it.
Set up BootManager, download it here. Then grab the ROM you want and put the zip file on to your SD card.
When you first start up BootManager, it will tell you to back up your "boot.img" file, do that just in case something goes wrong. Then hit "Install ROMs" and pick the ROM from your SD Card. Make sure all three boxes are ticked "Wipe system", "Wipe data" and "Wipe cache". It will then do what it needs to do, this part may take a while. When it has done it should give you the option to boot into the ROM. Hit done, however we aren't ready to boot into it yet.
Step 2 - Install Google Apps and whatever else
Popular ROMs such as CyanogenMod don't come with Google Apps because of copyright issues. So you will need to download the Google Apps zip file for the ROM you installed and go back to BootManager. Hit "Install ROMs" and pick the Google Apps zip file which should be on your SD card, and use the same slot you used in step one. Leave the three boxes unticked and then just let it do its thing.
You can now boot into the ROM.
Also with this method you can add themes and ROM add-ons.
Carry out steps 1 and 2 to add more ROMs.
Step 3 - Switch between ROMs
To boot into one of the other ROMs you have installed just go into BootManager and choose "Boot Installed ROM". It'll show you the list of ROMs you have and you just choose which ever one you want.
And there you have it, you can now have multiple ROMs on your Android device.