SharePoint 2010 Limits, Thresholds, and Boundaries

Here are some key SharePoint 2010 Boundaries and Limits
Via: http://technet.microsoft.com/en-us/library/cc262787.aspx

Highlights

Content database size - 200 GB per content database

Site collection size - 100 GB per site collection

File Size - 2GB - The default maximum file size is 50 MB. This can be increased up to 2 GB, however a large volume of very large files can affect farm performance.

Documents - 30,000,000 per library

Major Versions - 400,000

Bulk Operations - 100 items per bulk operation

List view lookup threshold - 8 join operations per query

List view threshold - 5,000

List view threshold for auditors and administrators - 20,000 - This setting works with Allow Object Model Override.

Subsite - 2,000 per site view

Web Parts - 25 per wiki or Web part page

Users in a site collection - 2 million per site collection

Alerts - 1,000,000 per search application - This is the tested limit.

Keywords - 200 per site collection

Social tags, notes and ratings - 500,000,000 per social database

Blog Posts - 5000 per site