A HighRankings forum thread named Can Google Index Unlinked Pages?, Can Google index unlinked pages? discusses ways to get indexed in Google without having any links that you know of.
Here are some of the answers in the thread:
- Using Google Add URL Form
- The URL appears up as a referrer in a pubic referrer log, or in trackbacks, etc.
- Google crawls ISP logs
- The Google Toolbar
- Google Sitemap
- Google Web Accelerator
- Google VPN
- Port 443 (SSL Certificate Scan)
- Register .gov, .mil, .edu domain.
Forum discussion at High Rankings Forums.