well there are a few ways to get free memory. the most useful for this widget would be

head -n 2 /proc/meminfo | tail -n 1

this will give you free mem in one line.

More informative is the command free that will give you more info on how it is allocated.

the command I gave is chopping one line out of the meminfo file. from an xterm type

cat /proc/meminfo

To see it all.

the field I have taken above is memfree. But I do not think that is very useful due to the way linux uses memory. Someone who knows more about the linux memory model nay be able to provide more useful info.