1) set other accounts up to forward to your gmail/gapps email 2) setup a filter + auto label system for those emails 3) add it as an outgoing email that you can reply from (limited to 7 i THINK)
Gmail is a class act when it comes to web apps.
I just use it as an offline backup, and keep it synced.
All my mail is synced locally but I can still use the Gmail interface.
4) Use Mailplane (http://mailplaneapp.com/). Disclaimer: Mailplane is not free and I'm not affiliated with it at all.
For example, if you're logged in as somebody@gmail.com, but send the message as evilperson@gmail.com, you'll reveal somebody@gmail.com to the recipient. In fact, Outlook proudly displays this address by stating "From: evilperson@gmail.com on behalf of somebody@gmail.com". I think you can also easily get this info from the mail headers from most web-based clients.
Maybe this isn't an issue for most folks, but it can create havoc with your filters if folks start replying to an address you had hoped to keep hidden from them.