How does Google find new URL/pages, except addurl and web links?

Alternative methods Google could use to find new URLs.

[LowLevel]

I was thinking about the methods Google could use to catch new URLs/domains, not already known by the search engine.

We all know about addurl form and links found on the web, but can you think about other ways Google could use to find new URLs or domain names?

Some examples:

URLs inserted in the search field
URLs catched by Google toolbar
Browser referrers
Domain names used in the e-mails they receive

[Sinner_G]

I'm not sure Googlebot is one of them, but some robots read log files. So if you have a link on your site to some site whose log file is spidered, it will show there as a referral and thus be found.

[creative craig]

Googleguy sitting there guessing them ;)

Craig