I use thunderbird but you can do what you asked in gmail.
The steps I believe are:
1. Forward All Email to 1 email address (you can use a filter to even forward the spam if you don't want false positives getting stuck in an email account you don't log in with often.
2. Set up the ability to send mail as all the email accounts on your main account (this is done in settings, i think it was under identities or something)
3. Select the option to (on reply) send email from the address it was sent to
With this you will also be able to send from any email account from your main one.
Thanks for the information, unfortunately one is a Gmail account (personal) and the other 2 are Google apps accounts (startup and side project) and I don't think you can send mail looking like a user at a Google Apps address.
Also I don't really want all my email from these 3 services mashed together into the one inbox as it'd be almost impossible to undo and it'll get messy really quick.
All I really want is a side pane in Gmail similar to Thunderbird / outlook which shows the 3 accounts and their folders with how much mail is in each folder, then I can easily click on any folder to navigate to it straight away.
Google has the capacity to make this really easy; the fact that they haven't makes me wonder how difficult it is to aggregate search and other data into one user profile, as that seems like the primary argument against easy Gmail (and thus Google) account mashups.
By far my biggest frustration as a 5-account, 3-OS (plus mobile) GMail and Apps user is the client. Two personal accounts, an academic and side project, and a professional one. Oy. Thunderbird provided some cross-platform consistency, but was far too clunky to last in my workflow - not surprised it died.
What I've done for the last six months (before recently experimenting again) was keeping the six accounts completely separate and using Sparrow on OSX/iOS as the primary means of access. It kept headers from getting all effed up with Gmail aliasing, but was a real pain when I would occasionally have to log in via web (signing in/out multiple times to check inboxes, etc.).
I've now decided to keep all non mission-critical forwarded to the same account (for me 5 Gmail/Apps accounts, for you would be your personal and side project accounts) and the professional account (Apps) separate. With Google allowing a max of Gmail and one Apps account at the same time, this works fine, so long as I'm willing to have my headers display stupid "sent by [me] on behalf of [my other email]" text. Which I am at this point for a little simplicity.
Agreed that an aggregator/navigator plugin would be killer (even if it just forwarded you to an incognito tab with your mailbox, to keep you from having to sign out of your main Google account) but the implementation details are onerous/likely impossible.
The steps I believe are:
1. Forward All Email to 1 email address (you can use a filter to even forward the spam if you don't want false positives getting stuck in an email account you don't log in with often.
2. Set up the ability to send mail as all the email accounts on your main account (this is done in settings, i think it was under identities or something)
3. Select the option to (on reply) send email from the address it was sent to
With this you will also be able to send from any email account from your main one.