Two of my favorite system monitor widgets have been updated: iStat pro 2.3 and iStat nano 1.5. The main new additions are drive filtering, as in: CDs, DVDs and .DMGs will no longer appear in the drive list, and support has been included for MacBook Pro batteries. Unfortunately, it looks like the temp and fan monitors don't support the MacBook Pros just yet.
Both of these widgets are available from iSlayer.net, apparently for free, as I can't even find a donation link. Also: their download links do point to these new versions, so don't pay any attention to the versions listed on their site, as it seems they haven't updated it to reflect these new additions just yet.