This is rather specialist, but...
Here's an Excel add-in [download] that puts a 'Remove Low Numbers' option in the Tools menu. When run, this converts any cells that are equal to or less than zero to 0.0001.
This is useful if, for example, you do a lot of spectrophotometry or RT-PCR and have to plot curves of the data on a log scale, which would normally give an error for values of zero or less.