This makes use of Google's Calculator, which is pretty handy.
It can convert units as well as do simple math calculations.

Couple examples:
!gcalc 4 grams in tons
-Returns 4 grams = 4.40924524 × 10^-6 short tons

!gcalc 4*5
-Returns 4 * 5 = 20

Pretty simple. A note about the unit converter though, it *can not* do conversions if they are not in a similar type of measurements.

e.g. !gcalc 4 grams in gallons

I know it's possible to convert it longhand, but this script just retrieves google's response. on *:TEXT:!gcalc *:#: {
if ($sock(google)) { sockclose google }
set %chan $chan
sockopen google google.com 80
sockmark google $2-
}
on *:sockopen:google: {
sockwrite -nt $sockname GET $+(/search?hl=en&q=,$replace($sock(google).mark,$chr(32),+),&btnG=Search)
sockwrite -nt $sockname Host: google.com
sockwrite -nt $sockname $crlf
}
on *:sockread:google: {
var %x
sockread %x
if ($regex(%x,/<h2.+?<font.+?<b>(.+?)<\/b/i)) {
var %t $regml(1)
%t = $regsubex(%t,/&#(.+);/,$chr($regml(1)))
%t = $remove($replace(%t,<sup>,^),</sup>)
msg %chan $nohtml(%t)
unset %chan
sockclose google
}
}