According to Google Support you can use the recovery link on this page: Recovering sign-in information
From here, you should try with all accounts you are associated with to see what accounts are linked. (Assuming you remember all email accounts used apart from the one you forgot). It is sort of hit and miss but it has worked for me on two separate occasions.
You should get an email similar to the following.
Your recovery address, phwd@gmail.com, is associated with:
xxxx@gmail.com
xxxx@gmail.com
xxxx@gmail.com
xxxx@gmail.com
xxxx@gmail.com
To sign in, visit the link below.
http://www.google.com/accounts/
If you've received this mail in error, it's likely that another user
entered your email address by mistake while trying to recall their
username. If you didn't initiate the request, you don't need to take
any further action and can safely disregard this email.
If clicking the link above does not work, copy and paste the URL in a
new browser window instead.
Thank you for using Google.
For questions or concerns regarding your account, please visit the
Google Accounts FAQ at http://www.google.com/support/accounts/.
This is a post-only mailing. Replies to this message are not
monitored or answered.
As seen above it works for multiple accounts. So any account that has phwd@gmail.com listed is brought up in the list.
Additionally you can search your email accounts for the following subject line
Your Gmail address has been created
This will bring up (if you didn't delete it), the confirmation email messages of all Gmail accounts you used the current account as a recovery point. Although, based on "used in ages" I am not sure how far back Google used to send out this message.
GMail Mail Fetcher
Connect your ISP's account to your Gmail account using POP3 so your Gmail account will serve as email client that will read emails from another POP3 (optionally also SMTP) account. You will use the same connection settings as your ISP is providing you with to connect an email client (i.e. Outlook) to fetch email.
I'm using a similar thing and also set a rule that the other emails are getting labelled correctly so I immediately know which emails are actually POP'ed and which ones are addressed to by Gmail account.
Read this guideline how to regularly get email from another account to your Gmail.
Best Answer
Only the Google Labs are added from the Google applications settings, other applications should include a way to add themselves to the user account. I.E. Google Apps Script web application that require to access user data ask authorization to the user to access their private date on the first run.