Battery Left - Tells you % battery remaining AND estimated TIME your phone will be dead (e.g., 9:59 am) - Also puts in status bar
WatchDog Lite - Keeps eye on CPU usage and you can use it as a task killer, as needed. Recommended by a Lifehacker article that concluded Task Killers are at best not needed or at worst bad for your Android device.
Mini Info - Shows Battery Percent, remaining storage in phone and in SD (in GB) -- I also use it to quickly turn GPS on -- and as double check on Battery Left
Smart Taskbar - Allows you to get to ALL your apps (in organized fashion) from ANY of the 7 pages of your homescreen. This seems to help save battery too, as you don't have active widgets all over the place using data
GMAIL Notifier - I have multiple gmail accounts, and I use the Gmail app instead of Email app to get my gmail "pushed" [The Email app is fetch, if you didn't know]. Gmail notifier lets me customize status bar icons for EACH gmail account and assign different alerts to each.
-SD Mount Widget - mounts your sd card without a reboot if you download media/pix directly on the phone.
-SMS Reader - Reads aloud your SMS and incoming Caller ID
-Setting Profiles Full - set custom profiles based on your own rules
-Image Hunter - my friends don't like their pix taken, so I download icons to symbolize them in my address book
-Fast Reboot - reboots your UNrooted phone without physically powering off your phone. So instead of waiting 2 minutes for a power up, it only takes a few seconds.
Many more, but these are my "can't live withouts"...