I'm not sure if you can use f..oo@gmail.com but if your email address is longer than a few characters then there are definitely a large number of combinations you can use if you want to register multiple accounts at one service, foo@, f.oo@, fo.o@, f.o.o@ etc.

You can also use foo+extra@gmail.com and the +extra will be ignored, so you can give each service a separate email address like foo+hn@gmail.com.

Beware, as I found out the hard way, this makes account recovery more difficult. To reset your password you need to enter your email address usually. But if you used +foo stuff then you might not be able to remember what email address you used.