Firefox: Google Wave probably isn't part of your day-to-day routine yet, but it might helps to know when new waves are waiting for you. Google Wave Notifier does exactly that, checking for unread wave messages on an interval you prefer.
Chad Smith, author of the similarly helpful Google Voice Notifier, patched together this 0.0.1 Firefox extension from a rewrite of his Voice add-on, so there are definitely a few kinks to work out, especially if you use a Firefox apart from the mainstream build. Smith's Google Wave Add-On (as it's technically, generically titled) does exactly what it promises, though — stores your login credentials, checks for messages every X minutes, as you specify, and puts a number in your status bar if you've got unread waves.
Google Wave Notifier/Add-On is a free download, works wherever (mainstream) Firefox does.
Google Wave Add-on for Firefox [That Smith]