The Centre Finder ruler is one of the best tools around. If you need to locate the centre of anything, just place the same number on each edge of your project and the O marks the centre. Metric and imperial, 30cm long.
This fantastic set of three acrylic blocks, including handle, gives you all the sizes you'll need for use with clear and cling stamps. Simply place a stamp of your choice on your block and attach the handle to use.