Our Library e-resources blog recently described how you can now find full text resources from OAIster through our Discovery Service SEEKER. See the full details in this post.
OAIster harvests records from worldwide institutional repositories including Research@StAndrews:FullText and makes them easily searchable.
A similar service that allows you to search and find open access resources in repositories is BASE (Bielefeld Academic Search Engine). Outputs from a specific repository can be found by refining your search results in the Collection drop-down list.
The Directory of Open Access Repositories OpenDOAR also provides a search service for material in its directory at http://www.opendoar.org/search.php