Recently, the NHibernate team released NH 4.0. You can get it as a NuGet package from https://www.nuget.org/packages/nhibernate, as a ZIP from http://sourceforge.net/projects/nhibernate/ or as source code from https://github.com/nhibernate/nhibernate-core. The full release notes are available at https://github.com/nhibernate/nhibernate-core/blob/master/releasenotes.txt, but here are some highlights.
One of its most important features is that it no longer needs Iesi.Collections for most common uses: where you would declare a set as Iesi.Collections.Generic.ISet<T>, now you use System.Collections.Generic.ISet<T>. However, if you want to use the order-by attribute of sets, you still need Iesi.Collections, because NHibernate will still use it (LinkedHashSet<T>). All of the public APIs that used Iesi.Collections.Generic.ISet<T> or Iesi.Collections.ISet now use System.Collections.Generic.ISet<T>, this includes all the event handler methods.
Method SaveOrUpdateCopy of ISession was removed and Merge should be used instead.
The ManagedWebSessionContext context class was removed and the managed_web alias for XML configuration is no longer supported. Now you should use WebSessionContext and web.
The Firebird driver was removed and replaced for FirebirdClientDriver. ASA10ClientDriver, ASAClientDriver and SQLiteDriver were removed in favor of SybaseSQLAnywhereDriver, SybaseAsaClientDriver and SQLite20Driver, respectively. A managed ODP.NET driver for Oracle is now available (OracleManagedDataClientDriver).
Sql2012Dialect was introduced and now properly pages records. Also added Ingres9Dialect. SybaseASADialect (use SybaseSQLAnywhere10Dialect),
MySQL now supports batching inserts (MySqlClientBatchingBatcher).
There have also been some breaking changes in the IProjection interface because the IEnhancedProjection interface was removed and its methods moved to IProjection.
Misspelled class AdoNetWithDistrubtedTransactionFactory is now AdoNetWithDistributedTransactionFactory.
NHibernate XML configuration on App.config or Web.config now accepts attribute linqtohql.generatorsregistry.
On mapping by code, you can now specify the class of a one-to-one endpoint (IOneToOneMapper.Class) and an optional table check clause (IClassMapper.Check). Also available are enhanced id generators Enhanced Sequence (Generators.EnhancedSequence) and Enhanced Table (Generators.EnhancedTable). You can now pass a dictionary to IGeneratorMapper.Params, no need to use anonymous types, which is better for dynamic code.
QueryOver projections now support Math.Round() to round up floating point numbers.
DefaultIfEmpty is now supported with GroupBy and Distinct is now supported with anonymous types.
Finally, all static type fields in NHibernateUtil are now declared as their exact type, not a base class, which is better for using in mapping by code.