If you've got multiple Gmail accounts, such as different Google Apps Gmail accounts (for hosted domains) and gmail.com accounts, being able to forward them all to one account saves a lot of time and effort. Gmail does allow you to forward email under the Forwarding and POP/IMAP section of the control panel, however this only forwards non-spam emails. As most people know, sometimes the spam filters aren't quite perfect and suffer from false positives occasionally.
To forward all mail to another account, simply remove your existing forward, create a new filter from * (matches any sender), then forward it to your main email address - making sure you've got Never send to spam selected. You'll probably want to choose Delete it as well, this means any incoming mail on that specific account will be forwarded and deleted on the original account - but you'll still have 30 days until it gets removed from trash.
This way you'll get all mail forwarded from Gmail - including spam. Now you can see all the spam from any other Gmail accounts incase there are any false positives.