byobu on my home server, and I noticed this today - what is it?
It's part of the byobu notification/status bar, on a 64-bit 11.10 server.
From man pages:
updates_available - the number of updates available on the system; displayed in the lower bar toward the right in white text on a red background with a trailing '!' sign; if any updates are marked 'security updates', then there will be a total of two trailing exclamation points, '!!'
This shows the number of packages that need to be updated on the system.