Suppose your email address is 'gogletruths@gmail.com', basically everything sent to any of the following email addresses will be forwarded to your primary email.

gogletruths@gmail.com [Original ID]
gogle.truths@gmail.com [Secret ID]
go.gle.tru.ths@gmail.com [Secret ID]
g.o.g.l.e.t.r.u.t.h.s@gmail.com [Secret ID]
gogletruths@googlemail.com [Secret ID]
go.gle.tru.ths@googlemail.com [Secret ID]


And that's not all, you can place as many dots as you want, it can be even something like 'g.o.g.l.e.t.r.u.t.h.s@gmail.com' and you'll still get it on 'gogletruths@gmail.com'.

Additionally, I just also found out that you can embed random text to your email ID using '+' sign. That is to say 'gogletruths+hacking tool@gmail.com' can be used as your email address, as well.

gogletruths+hacking_tool@gmail.com
gogletruths+information_provider@gmail.com
gogletruths+ What Many People Do Not Know About Google@gmail.com

So, instead of using different email addresses for various purposes (work, school, friends, etc.) you can use different variations of your Gmail and filter incoming mails by 'sent to' address.