This is a somewhat older question and the correct answer has changed with recent versions of QGIS.
PostGIS Manager has been deprecated, and superseded by DB Manager which works for both PostGIS and SpatiaLite.
DB Manager can easily be installed using the plugins manager. Once installed it is accessible under the Database > DB Manager menu entry. That will open up the DB Manager window.
Select the database you want to add a layer too and then click Table > Create table. If you don't have a database selected when you click on the Table menu entry, the create table option will not be there.
That will open up the create table dialog which is very similar to the one accessed by Layer > New > New SpatiaLite Layer but with a different layout.
The options are mostly the same with the largest difference being that the New Spatialite Layer dialog allows access to the Specify CRS dialog which makes selecting your coordinate system easier. The Create table dialog requires that you know the SRID number beforehand to enter it.