SeaMonkey 2.0 Alpha 2 contains the following major changes relative to SeaMonkey 2.0 Alpha 1:

• The first step towards customizable toolbars has been made: The icon size and whether to display icons and/or text can be chosen spearately for every major toolbar in in both the browser and MailNews components via their context menu.
• The MailNews component now includes support for reading RSS and Atom feeds, And the browser detects feeds offered by web pages. Since the support for previewing feeds in the browser has not yet been included, the feed icon at the right end of the URL bar currently is directly invoking subscription to the feed in the MailNews component, if an account for feeds has already been created there.
• Tracemonkey, the Just-In-Time (JIT) JavaScript compiler, has been enabled for web content by default, resulting in a significant speedup of scripts running on web pages.
• The Find As You Type functionality has been internally reworked to use the same code behind the scenes as the Firefox find bar, but keeping the function as presented to users unchanged from SeaMonkey 1.1.

See the SeaMonkey Trunk Tracker blog for more details about the changes between those pre-releases.
The changes page lists an overview of new features and fixes relative to our last stable release, SeaMonkey 1.1.13.