I'm using:
- Google Calendar Gadget - it is nice to have a sneak preview of what the day looks like
- Google Docs Gadget - I write technical documents a lot and switch back and forth with other tasks. This is a quick access to the latest documents you have been working on
- Quick links - Do you have a task in Google Apps that requires you to go through a few clicks to get there? Or do you do frequent searches for something? By adding the URL of the Google App page of what you want to get to the Quick links will allow you to get that with just one click.
- Signature tweaks - I like my signature right after my reply, not at the bottom of the email and after all quoted messages as it is by default. This lab tweak will bring the signature up to the top of the email.

There are other features that I miss a lot from MS Outlook; one of them is mentioned by "ziffdavis" which is the re-send option. Other features I would like to see are:
- Sort messages by date, sender or subject
- Labels for calendar items - they exist for emails, why not for calendar?
- Sorting contacts by company, and that this field is visible in the list of contacts
- Attach a message to a message - I used this feature a LOT in Outlook; whenever I needed to remind someone of a previous email where we had a discussion about something I would just attach the email to the new one.

I can't remember other things I have missed but that's the bulk. I know I went on talking about not just email features but since these Apps are all integrated I though I might point them as well.
Thanks,